Adults, Adolescent Differential Treatment

Developmental Psychology Copyright 1985 by the American Psychological Association, Inc. 1985, Vol. 21, No. 5, 747-760 0012-1649/85/$00.75

Differential Experience of Siblings in the Same Family

Denise Daniels and Robe r t P lomin Institute for Behavioral Genetics

University of Colorado

Much attention has recently been drawn to the possibility that children within the same family experience different environments. The present study investigated dimensions of differential experience reported by siblings in the same family and examined the origins of these experiences. The Sibling Inventory of Differential Experience (SIDE) was completed by 396 adolescent and young adult siblings from both adoptive and nonadoptive homes. The SIDE asks each individual to compare his or her experiences to those of one of his or her siblings in the domains of sibling interaction, parental treatment, peer characteristics, and events specific to the individual. The results indicate that, on the average, siblings experience quite different environments, especially in the areas of differential sibling interaction and differential peer characteristics. Differential sibling experience as assessed by the SIDE shows little genetic influence, which implies that its origin is primarily environmental.

The traditional approach to studying the relationship between children's environments and their development assumes that children in the same family are exposed to similar environmental influences, such as socioeconomic opportunities, child-rearing attitudes, and other parental characteristics. For this reason, various family environmental measures such as parental affection, control, and responsive- ness; number of toys; intellectual stimulation provided; and authoritarian treatment have been scored in relation to one child per family and compared across families. These between- or across-family environmental measures are related to adjustment, personality, and intelligence scores of one child in the family. This traditional between-family approach to studying behavior-environment relationships has been used extensively at the expense of ne-

We wish to thank the families who participated in this study, and we gratefully acknowledge the assistance of Lutheran Social Services of Colorado in helping us contact the adoptive families. This article was written while Robert PIomin was supported by a Research Scientist Development Award from the National Institute on Alcohol Abuse and Alcoholism (AA-00041). We appreciate the excellent ed- itorial work of Rebecca Miles and the manuscript prepa- ration skills of Dianne Johnson.

Requests for reprints of this article or for copies of the Sibling Inventory of Differential Experience questionnaire should be sent to Denise Daniels, Institute for Behavioral Genetics, Campus Box 447, University of Colorado, Boulder, Colorado 80309.

glecting within-family environmental influences. In a recent review of family socialization research, Maccoby and Martin (1983) pointed out that behavior-environment relationships found with the between-family approach usu- ally have been small and that most of the variance lies within, rather than between, families. Low-order resemblance between siblings suggests that environmental influences relevant to psychological development largely operate in such a way as to make siblings in the same family different from, rather than similar to, each other (Rowe & Plomin, 1981). From analyses of familial resemblance, within-family variance has been estimated to account for approximately 30% to 50% of the environmental variance in cognitive traits (McCall, 1983) and approximately 80% to 100% of the environmental variance in personality traits.

Until recently, studies of differences within • a family have not gone beyond the study of family constellation variables. Although fre- quently studied, the effects of birth order, age, and sex of siblings do not tell us much about personality traits and cognitive abilities. For instance, birth order and sex of a child explain

• only 1% to 10% of the variance in achievement and ability scores (Plomin & Foch, 1981; Scarr & Grajek, 1982). Furthermore, when socioeconomic status is controlled and when siblings from the same family are compared, birth order-behavior relationships approach zero (Ernst & Angst, 1983).

747

748 DENISE DANIELS AND ROBERT PLOMIN

In a first study of the relationship between environmental differences within the family (other than family constellation variables) and sibling behavioral differences, Daniels, Dunn, Furstenberg, and Plomin (1985) analyzed data on a nationally representative sample of 346 adolescent sibling pairs. It was found that the sibling who experiences more maternal closeness, more say in family decisions, and more peer and sibling congeniality, as reported by both the parents and the siblings, also shows better psychological adjustment as reported by parents, self, and teachers. The Daniels et al. study was limited in that the within-family measures were created indirectly from between-family measures scored individually for each sibling. Nevertheless, this study suggests that nonshared family environmental influences can be assessed and do show systematic relationships to behavioral development. Al- though the relationships between sibling adjustment differences and environmental differences within the family were small, they were essentially independent of the effects of birth order, age, and the sex of the siblings.

As much as 30% to 100% of the environmental variance and 15% to 70% of the total phenotypic variance in personality, cognition, and psychopathology lies within families (Rowe & Plomin, 198 l). However, researchers have only begun to examine nonshared family environmental influences. The present study was designed to investigate the microenvironments experienced by each individual in the family by directly assessing and identifying differential experiences of siblings in the same family and by examining the origins of these experiences. These goals represent the first crucial steps in investigating nonshared family environmental influences.

Assessment of Differential Sibling Experience

Rowe and Plomin (198 l) developed a con- ceptual framework to investigate differential sibling experience. This framework includes sibling interaction, parental treatment, extra- familial network influences, and experiences such as accidents or death of a loved one that are likely to be specific to one individual. It was decided that this initial study should assess all of these domains, because there were few

leads as to which are the most important sources of differential experience. To this end, we developed the Sibling Inventory of Differ- ential Experience (SIDE) questionnaire in which each individual is asked to compare his or her experiences to those of one of his or her siblings.

The wide variety of questions that may be addressed using the SIDE include the following: What percentage of siblings have nonmutual feelings of jealousy or closeness? To what extent do parents favor or punish one sibling more than the other? What are the chances that one sibling will join a more delinquent or more popular peer group? What percentage of siblings view life-threatening events or the influence of a teacher for one sibling and not the other as a source of sibling differential experience? We assessed the amount of sibling differential experience for each domain included in the SIDE--nonmutuality of sibling interaction, differential parental treatment, differential peer group characteristics, and events specific to an individual. The study also examined psychometric properties of the SIDE.

Origins of Differential Sibling Experience

In addition to assessing the differential experience of siblings, we explored the origins of these experiences. Specifically, we investigated the extent to which genetic differences between siblings or sibling differences in family constellation variables are responsible for differential experience.

It has recently been discovered that standard environmental measures are influenced genetically (Plomin, Loehlin, & DeFries, 1985; Rowe, 1983). In terms of within-family environmental influences, parents might restrict one sibling more than the other because of genetic differences between the siblings. Knowl- edge about genetic influences on the experiences assessed by the SIDE will help to address the question of cause and effect in future in- vestigations of within-family environment- behavior relationships. Specifically, if the SIDE measures are found not to be genetically influenced, the data would not support the hy- pothesis that genetic differences lead to expe- riential differences between siblings.

The extent to which differential sibling ex-

DIFFERENTIAL SIBLING EXPERIENCE 749

pe r i ence w i t h i n a f a m i l y is gene t ica l ly o r en- v i r o n m e n t a l l y i n f l u e n c e d is add res sed in the p re sen t s tudy by c o m p a r i n g d a t a on b io log ica l full siblings (who are 50% genet ica l ly different, on the average, for segrega t ing genes) a n d adop t ive s ibl ings (who, in t he absence o f se- lec t ive p l a c e m e n t , a re 100% di f ferent genet i - cally, on the average, for segrega t ing genes). I f he red i t a ry s imi la r i ty in s ibl ing behav io r affects the i r d i f ferent ia l expe r i ence , t h e n g rea te r dif- f e rences in e x p e r i e n c e shou ld be f o u n d for adop t ive pai rs t h a n for b io log ica l pairs. Col - l ec t ing d a t a f r o m twins is an a l t e rna t ive s trat- egy for assessing gene t ic i n f luence on differen t ia l expe r i ence , because the gene t ic differ- e n c e b e t w e e n iden t ica l tw ins is 0%, a n d tha t b e t w e e n f ra te rna l tw ins is 50%. However , because twins a re exac t ly the s a m e age a n d because iden t ica l tw ins a re exac t ly t he s a m e in a p p e a r a n c e , tw in resul ts c o n c e r n i n g dif feren- t ia l e x p e r i e n c e m a y n o t genera l i ze to n o n t w i n s ibl ings (Lykken , 1978). F u r t h e r m o r e , un l ike siblings, tw ins c a n n o t be used to assess the effects o f f ami ly cons t e l l a t ion var iab les such as b i r th order, sex, a n d age. Fo r these reasons , we used d a t a on adop t ive and b io log ica l s ibl ings to exp lo r e the or ig ins o f d i f ferent ia l s ibl ing expe r ience .

M e t h o d

Sample

The sample consists of 396 siblings of Caucasian ancestry from the Denver metropolitan area. The mean education of the siblings' fathers and mothers is 16.3 years and 15.0 years, respectively. Siegel Prestige Scores (Hauser & Feath- erman, 1977) were used to determine that the average socioeconomic status of their fathers is 54.9 (SD = 13.3). Although this average is about one standard deviation above the mean of the U.S. white labor force as determined by census data, the nearly identical standard deviations suggest reasonable representativeness in terms of the present sam- pie's variability (Siegel, 1971).

Adoptive families were identified through the Lutheran Social Services of Colorado, a Denver adoption agency. These families adopted children during the 1960s and 1970s, and most of them include both adoptive and biological siblings. Biological sibling pairs obtained for our sample from these families were supplemented by con- tacting students at the University of Colorado and their siblings. Nearly 600 families initially were asked to participate in the study. One third of the adoptive families could not be contacted because they had moved from the Denver area and their place of residence could not be traced; 247 families volunteered to participate; and the remaining families did not reply or declined. Our sample includes 149 of these families for which complete information is

available on two siblings (both completed the questionnaire) and another 98 families for which we have information about family constellation variables for two siblings, but only one completed the questionnaire. Information was collected through the mail. Siblings were instructed to work independently if both were still living at home and to leave questions unanswered if the question did not apply or if they did not know or remember the answer.

The siblings range in age from 12 to 28 years (M = 18.1, SD = 3.63). More females (219) than males (177) volunteered. The sample includes 171 adoptees and 225 biological siblings. Half of the families are two-child families, so most of the siblings (276) were first- and second-born children. The remaining 120 siblings were third-, fourth-, fifth-, sixth-, and seventh-born children. For the majority of analyses, siblings are conceptualized as belonging to a pair; even if only one sibling participated, the brother or sister who was rated by that individual is considered to be the other member of the pair. With regard to their genetic relationship, there are 222 pairs who are genetically unrelated and 174 who are 50% genetically related (full biological siblings). There are 190 brother-sister pairs, 124 sister pairs, and 82 brother pairs. Regarding birth order, there are 226 first-second born pairs, 61 second-third born pairs, 41 first-third born pairs, and 68 of the remaining variety of birth orders (first-fourth, fifth-sixth, etc.).

The SIDE

The questionnaire that we developed to ask siblings to compare their experiences is known as the Sibling Inventory of Differential Experience (SIDE). The SIDE and detailed psychometric data are available from the first author.

Construction of the SIDE was a three-step process. First, items were created or were selected from well-known ques- tionnaires on between-family influences (Dibble & Cohen, 1974; J. Dunn, personal communication, September 1982; Leary, 1957; Loehlin & Nichols, 1976; Schaefer, 1965) to assess sibling interaction, parental treatment, peer characteristics, and events specific to an individual. Second, all items were revised to ask siblings to compare their relative experiences rather than to make absolute judgments of their environment. For example, a between-family item such as "my sibling and I show understanding for each other" was changed to "Who has shown more understanding for the other?" Third, the measure was pilot tested in a study of 50 university students. On the basis of prelim- inary analyses, as well as feedback from the pilot subjects, certain items were deleted and others were added.

Although the SIDE explores a broad domain of experiences that may differ between siblings, it is limited in certain respects. For example, it focuses on social-affective rather than cognitive experiences. In addition, it is based on sibling perceptions rather than on observational as- sessments. Finally, it should be noted that each question of the SIDE is phrased so that individuals respond by averaging over the years when they were growing up and riving at home. For example, a question is phrased, "In general, who has been more bossy toward the other over the years?" rather than "Who is more bossy toward the other?." The decision to ask questions in this manner was based on feedback from our pilot subjects who often indicated, for example, that their sibling had been bossy toward them until recently. By asking the subjects to average over the


years, the data obtained in this first study on differential experience within the family are comprehensive as a start- ing point. It should be noted that in future studies the SIDE questions could easily be changed to refer to any time period (such as the past month).

Following are descriptions of the four categories of differential experience as assessed by the SIDE and an ex- planation of scoring procedure.

Differential sibling interaction. The SIDE contains 24 items that assess differential sibling interaction. The questions tap four underlying factors: antagonism, caretaking, jealousy, andcloseness. For any of these four dimensions, interactions between siblings could range from mutual to nonmutual. For instance, both siblings could interact co- operatively and in a caring manner, thereby providing similar environments for each other. On the other hand, one sibling could act in a kind and understanding way toward the other, whereas the other sibling might reciprocate with angry and jealous behavior, thereby creating different environments for the two siblings.

Differential parental treatment. The SIDE includes nine items on differential parental treatment, answered separately for mothers and fathers, which assess two main factors: affection and control. For each of these two dimensions, siblings could report that they are treated equally, a bit differently, or very differently. In the pilot study, we also had included items about intellectual stimulation, expec- tations to achieve, supportiveness, and protectiveness; however, they were dropped from the final version of the SIDE for the following reasons: Questions about intellectual stimulation yielded little variance (80% of the siblings reported their parents treated them similarly). Differential parental expectation items did not duster together in factor analyses. The factors of differential parental supportiveness and differential parental protectiveness yielded low test- retest reliability.

Differential peer characteristics. The SIDE Contains 26 items concerning differences between the siblings' peer groups. The questions refer to three dimensions of peer group characteristics: orientation toward College, delinquency, and popularity. The original version of the questionnaire had asked about a much greater variety of peer interests--including outdoor activities, art, domestic activities, television and video games, cheerleading, sports, and music. However, because these varied interests did not cluster systematically in factor analyses, we dropped them from the SIDE.

Events specific to the individual. Unlike the other three categories of differential experience, the fourth category consists of 14 items that may be unique to one or the other of the siblings. Siblings are asked about the impact of boyfriend and girlfriend relationships, relatives, friendships, teachers, accidents, divorce, meeting a special person, ex- traordinary events, death of a loved one, and family psychological problems.

Scoring o f the SIDE. Each SIDE item is answered on a 5-point scale. For example, one of the sibling interaction items, "In general, who has been more willing to help the other succeed over the years?'; is rated as follows: 1 = my sibling has been much more this way than I have, 2 = my sibling has been a bit more this way than I have, 3 = my sibling and I have been the same in this way, 4 = I have been a bit more this way than my sibling, and 5 = I have been much more this way than my sibling. Similarly, if

asked about parental treatment (e.g., "mother has been sensitive to what we think and feel"), the responses are as follows: 1 = toward sibling much more, 2 = toward sibling a bit more, 3 = same toward my sibling and me, 4 = toward me a bit more, and 5 = toward me much more. Similar rating scales are used for peer group characteristics and events specific to the individual.

These 5-point rating scales lead to relative scoring of differential sibling experience. In other words, the scaling procedure provides information concerning the amount and the direction of differential experience--which sibling perceives more conflict and how much more, which sibling feels less parental love and how much less, which sibling believes his peers are more popular and how much more, and so forth.

Also of importance is the absolute amount of sibling differential experience in which the direction of differential experience is disregarded. Each relative response can be recoded on a 3-point absolute scale on which a relative score of 3 is coded as 0 (no difference in sibling experiences), relative scores of 2 and 4 are coded as 1 ("a bi t"o f difference in sibling experiences), and relative scores of 1 and 5 are coded as 2 (much difference in sibling experiences).

Analyses After examining the psychometric properties of the

SIDE, we assessed the amount of the differential experience of the siblings as reported in the SIDE. We then explored the origins ofdifferential sibling experience by comparing data on the adoptive and biological siblings and by examining the relationship of their differential experience to family constellation variables.

Resu l t s

P s y c h o m e t r i c P r o p e r t i e s o f t h e S I D E

F a c t o r a n a l y s e s o f S I D E i t e m s . Rela t ive scores on i t ems in the first th ree categor ies were s u b m i t t e d to fac tor analys is us ing usua l tech- n i q u e s o f p r inc ipa l c o m p o n e n t f ac to r ing a n d v a r i m a x ro ta t ion . S ib l ing i n t e r a c t i o n i t e m s y ie lded fou r factors t ha t we labe led Di f fe ren- t ia l S ib l ing A n t a g o n i s m , Di f fe ren t ia l S ib l ing Care t ak ing , Di f fe ren t ia l S ib l ing Jea lousy, a n d Di f fe ren t ia l S ib l ing Closeness . T h e w o r d d/f- f e r e n t i a l precedes the label for each fac to r because all i t e m s invo lve re la t ive (or di f ferent ia l ) ra t ings: Fo r e x a m p l e , i f an i nd iv idua l r epo r t s tha t he o r she is m o r e l ikely to s ta r t fights as c o m p a r e d to the sibling, t ha t i nd iv idua l also t ends to r e p o r t t ha t he o r she is m o r e s t u b b o r n t o w a r d his o r he r sibling. Pa ren t a l t r e a t m e n t i t e m s y ie lded two factors tha t we have n a m e d Different ia l Paren ta l Affec t ion and Dif ferent ia l Pa ren ta l Con t ro l . Peer cha rac t e r i s t i c i t e m s y ie lded t h r ee fac tors tha t we call Di f fe ren t ia l Peer Col lege O r i e n t a t i o n , Di f fe ren t ia l Peer

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Delinquency, and Differential Peer Popularity. The factor structures were similar when examined separately by sex, adoptive status, and age. Scores on items specific to an individual were not factor analyzed because, by defini- tion, the items refer to idiosyncratic events and thus are not expected to cluster together. The results of the factor analyses are shown in Table l for differential sibling interaction items, in Table 2 for differential parental treatment items, and in Table 3 for differential peer characteristic items.

Construction of SIDE scales. The results in Tables l, 2, and 3 were used to create scales of differential experience. The scales were created by reversing the items that load negatively on a factor; summing the items; and dividing by the number of items. For instance, the Dif- ferential Sibling Antagonism Scale was created by first reversing Items 13, 15, and 20, then by summing Items l, 7, 9, l l , 13, 15, 16, 18, and 20, and, finally, by dividing by nine (the number of items). The construction of SIDE

scales is explained in greater detail in the method section.

Descriptive information on SIDE scales. Means and standard deviations for each SIDE scale are listed in Table 4 for both relative and absolute scores. The means of the absolute scores center around 0.75 (a score of 1.00 indicates "a bit of difference") and is discussed later. The means of the relative scores for all the SIDE scales center around 3 (a score in- dicating no differential sibling experience), which is to be expected because scores of 4 and 5 indicate "self more than sibling," and scores of l and 2 indicate "sibling more than self." Importantly, the standard deviations range from 0.4 to 0.9, which means that siblings in many families perceive their experiences to be quite different.

Intercorrelations, test-retest reliability, and sibling agreement for SIDE scales. Intercor- relations among the SIDE scales and the test- retest reliability of each scale are shown in Ta- ble 5 (relative scores were used for all analyses

Table 1 Loadings of SIDE Items on Sibling Interaction Factors

SIDE item

Differential Sibling

Antagonism


Caretaking


Jealousy


Closeness

1. Start fights 7. Stubborn 9. Bitter

11. Anger 13. Understanding 15. Kind 16. Let down 18. Deceive 20. Get along 3. Show concern 4. Help to succeed 6. Take responsibility

12. Feel superior 19. Bossy 21. Supportive

5. Like to be with 10. Compare to 14. Jealousy 22. Try to outdo 23. Admire 24. Feel inferior 2. Trust 8. Confidence in

17. Show affection

.57

.57

.57

.67 -.45 -.65

.45

.36 -.50 -.30 -.34

.44

.36

.43

.53

.63

.62

.60

.44

-.37 - . 4 6

.31

-.32

.43

.67

.53

.43

.59

.51

-.43

.39

.39

.31

.49

.41

.44

Note. Only loadings of .30 and above are listed. SIDE = Sibling Inventory of Differential Experience.


Table 2 Loadings of SIDE Items on Parental Treatment Factors

Differential Differential Parental Parental

SIDE item Affection Control

26. Proud .54 27. Enjoy being with .68 28. Sensitive to feelings .38 30. Show interest in .66 32. Favoritism .59 25. Strictness 29. Punishment 31. Blame 33. Discipline

.67

.79

.33

.62


reported in the table). There are quite a few intercorrelations among the SIDE scales, yet they are low to moderate. Two-week, test-retest reliability is substantial--ranging from .77 to .93, with a mean of.84, for a sample of 57 biological siblings.

Sibling agreement on the SIDE scales is low to moderate for the 149 pairs in which both of the siblings completed the questionnaire (see Table 5). Because relative scoring was used, negative correlations on the SIDE indicate that the siblings are in agreement regarding their perceptions. For instance, if one sibling feels that the other sibling has been disciplined more by their father (scores of I or 2), the other sibling is also likely to feel that he or she was disciplined more (scores of 4 or 5). The median correlation between sibling perceptions of differential experience on the SIDE is -0.49, and all correlations are significant.

Amount of Differential Sibling Experience

A major goal of the study was to assess the extent of sibling differential experience with each other, with parents, with peers, and for events specific to the individual. Because this assessment involves the amount of differential sibling experience rather than the direction, the absolute scores on SIDE items were ana-

Table 3 Loadings of SIDE Items on Peer Characteristic Factors

Differential Peer Differential Peer Differential Peer SIDE item College Orientation Delinquency Popularity

35. Ambitious .55 38. Hardworking .49 39. Intelligent .72 40. Mature .63 43. Responsible .54 44. Successful .63 48. Well adjusted .43 49. College oriented .71 50. Achieve, school .72 51. Student government .5 l 54. Politics .70 58. Get along well .33 37. lazy 42. Delinquent 46. Rebellious -.31 47. Conforming 52. Partying 53. Drugs 57. Skip class 59. "'Bad" group - .32 34. Popular 36. Outgoing 41. Extraverted 45. Friendly 55. Achieve, status 56. Having a boyfriend/gidfriend

-.42 -.31

.44

.76

.73 - .36

.79

.80

.77

.78

.30

.73

.65

.57

.34

.70

.55


DIFFERENTIAL SIBLING EXPERIENCE

Table 4 Means and Standard Deviations for SIDE Scales

753

Relative scores Absolute scores

SIDE scales M SD M SD N

Differential Sibling Antagonism 2.90 0.62 0.79 0.37 326 Differential Sibling Caretaking 2.62 0.71 0.83 0.34 366 Differential Sibling Jealousy 3.02 0.77 0.86 0.44 333 Differential Sibling Closeness 3.09 0.71 0.71 0.47 382 Differential Maternal Affection 3.00 0.41 0.50 0.45 356 Differential Maternal Control 3.14 0.68 0.63 0.50 344 Differential Paternal Affection 3.00 0.45 0.52 0.46 354 Differential Paternal Control 3.11 0.70 0.66 0.52 349 Differential Peer College Orientation 3.32 0.63 0.78 0.38 287 Differential Peer Delinquency 3.04 0.88 0.87 0.50 224 Differential Peer Popularity 3.22 0.78 0.84 0.46 319

Note. SIDE = Sibling Inventory of Differential Experience.

lyzed. We began with analyses at the item level and averaged over the SIDE items in each category to determine the percentage of siblings who report similar or different experiences. The results are portrayed in Figure 1.

For the 24 differential sibling interaction items, 33.6% of the siblings on the average report mutuality of experience, 47.7% report "a bit" of difference, and 18.7% report much difference. Averaging over the nine parental treatment items (data for mothers and fathers combined), 56.5% of the siblings report similar parental treatment, 34.5% "a bit" of difference, and 9.0% report much difference. For the 26 peer characteristic items, 37.4% report similar peer group characteristics, 42.3% have peer groups that are "a bit" different, and 20.3% have peer groups that are much different. Fi- nally, for the 14 events specific to an individual, 36.8% report that they have been similarly exposed to them, 39.4% say they have been "a bit" differently exposed to these special hap- penings, and 23.8% say there has been much difference in their exposure to specific events. These results indicate that each domain of proposed differential experience is indeed a within-family environmental infuence: 40% to 65% of the sample report that their experiences over all categories differ to some extent from their siblings' experiences. More similarity is reported for parental treatment than for the other categories of differential experience.

Results of analyses of scores on the SIDE scales support this finding (see Table 4). Means of the absolute scores (0 = no difference in

sibling experiences, 1 = "'a bit" of difference in sibling experiences, 2 = much difference in sibling experiences) are 0.80, 0.50, and 0.83 for the sibling interaction, parental treatment, and peer characteristic categories, respectively. In accord with the data presented for SIDE items, these results for the scale scores suggest that sibling interaction and peer group characteristics are more influential sources of differential experience than is parental treatment. For all the scales, the standard deviations indicate considerable variability in differential sibling experience, suggesting that siblings in some families share similar environments, whereas other siblings perceive their experiences to be quite different.

Origins o f Differential Sibfing Experience

To test for genetic influence on differential sibling experience, mean scores based on absolute scoring of the SIDE scales were compared for biological siblings (those who compare themselves to a genetically related sibling) and adoptive siblings (those who compare themselves to a sibling who is not genetically related). Mean scores and standard deviations are presented in Table 6. The finding of higher mean scores for adoptive than for biological siblings suggests genetic influence. Samples of this size have 80% power to detect mean differences that account for 2% of the variance.

For four of the SIDE scales, adoptive siblings are significantly more likely to perceive their experiences as being different. This finding also

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emerged for adoptive same-sex siblings versus biological same-sex siblings and for adoptive opposite-sex siblings versus biological opposite- sex siblings. One of these four scales is Differ- ential Sibling Closeness. To a greater extent than biological siblings, adoptive siblings perceive nonmutual trust, confidence, and affection. The other three significant differences in- volve peer characteristic scales. Genetic influence is implicated in the extent to which one sibling is more likely than the other to be in a college-oriented peer group, a delinquent peer

group, or a popular peer group. Although genetic influence is implied by the significantly higher mean scores for adoptive as compared to biological siblings, it should be noted that only a small amount of the variance is gene t i c - t he mean adoptive-biological differences explain only 2%, 2%, and 6% of the variance, respectively, for the three peer scales. In general, the mean scores for the two types of siblings are similar: The averages are 0.69 and 0.76 for adoptive arid for biological siblings, respectively. Overall, the SIDE scales do not

Figure 1. Average percentages of differential sibling experience for the four categories of SIDE items.


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show much genetic influence, which implies that the origin of differential sibling experience is environmental.

Table 6 also shows mean scores and standard deviations based on absolute scoring of the SIDE scales for same-sex sibling pairs and opposite-sex sibling pairs. Opposite-sex siblings perceive significantly more differential experience than do same-sex pairs for 2 of the 13 scales: Parental Control and Peer Popularity. However, in general, same-sex and opposite- sex siblings report differential experience to the same extent; thus, sex has little effect on these perceived differences.

The effects of birth order and age on differential sibling experience were assessed by examining correlations of these family constellation variables with relative scores on the SIDE scales. For the differential sibling interaction scales, significant correlations emerged for birth order and age as they related to three of the four SIDE scales: Sibling Jealousy, Sib- ling Caretaking, and Sibling Closeness. Youn- ger siblings are more likely to feel jealous of older siblings (birth order, r = .27; age, r = - .20) and to feel close to them (birth order, r = . 11; age, r = - . 19). Older siblings are more likely to be caretakers of their younger siblings (birth order, r = - .30; age, r = .23).

In the domain of differential parental treatment, neither birth order nor age is related to differences in relative scores on the parental treatment scales. Slight but significant correlations emerged for two of the three differential peer characteristic scales: Birth order is related to Differential Peer College Orientation, .with earlier born siblings being more likely to belong to a college-oriented peer group (r = - . 12). Age is associated with Differential Peer Pop- ularity in that younger siblings are more likely to belong to a popular peer group (r = - . 18).

Sex, birth order, and age are constellation variables that differ for the siblings. Although these variables influence differential sibling experience to some extent, they account for no more than 10% of the variance on the SIDE scales. Other family constellation variables-- birth spacing, family size, and parental edu- ca t ion-a re common to the siblings. We found no more than a chance number of significant correlations between these common variables and the SIDE scales.

Finally, the question as to whether the absolute amount of sibling differential experience varies with developmental stage was investigated. The sample includes adolescent siblings still living at home (12 to 17 years of age) and young adult siblings living apart from each other (18 to 28 years of age). Only one significant mean difference appeared when these two groups were compared. (This is not surprising because all subjects were asked to respond by averaging over the years when they were living at home•) Younger siblings still living at home tend to perceive more differences on the sibling closeness scale than do older siblings who are both living away from home• This developmental stage difference accounted for 3% of the variance and also was confirmed in a quantitative analysis of the correlation between absolute amount of Differential Sibling Close- ness and age.

Discussion

The present study assessed dimensions of differential experience reported by siblings in the same family and examined the origins of these experiences. The study was prompted by recent evidence (Daniels et al., 1985; Dunn, 1983; Lamb & Sutton-Smith, 1982) and theo- retical perspectives (McCall, 1983; Rowe & Plomin, 1981; Scarr & McCartney, 1983) in- dicating that, although biological siblings share a common rearing environment and 50% of their segregating genes on the average, siblings in the same family in fact differ considerably from each other. Genetically related children growing up in the same family show correlations of .40 to .50 for cognitive abilities and • 15 to .25 for personality traits.

A large sample of adolescent and young adult siblings reported differences and similarities in their experiences by completing the SIDE. Psychometric information on the SIDE indicates that it is a reliable measure of sibling differential experience. It assesses four domains: nonmutuality of sibling interaction, differential parental treatment, differential peer characteristics, and events specific to each sibling. In each of these domains siblings experience different environments• In addition, there is marked variability in each domain in the extent to which siblings perceive their ex-


periences to be different. The origins of differential sibling experience were examined by comparing adoptive and biological siblings. These comparisons suggest that differential sibling experience, with the possible exception of peer group differences, is primarily environmental in origin.

This study has demonstrated that siblings, on the average, experience different environ- merits in a wide variety of areas; however, five caveats are in order. First, the results are based on subjective perceptions of adolescent and young adult siblings. The use of parental reports or objective observations or studies of siblings of a different age might yield different results. For example, in our pilot work using parental reports of differential sibling experience, parents generally responded that they do not treat their children differentially. However, Daniels et al. (1985) found that parents do perceive some differences in their treatment of siblings. They also pointed out that "the fact that adolescents perceive differences within their family regardless of whether the differences really exist is important in its own right."

Second, because the SIDE primarily assesses differential social-affective experience, our findings may not generalize to cognition. It could be that cognitive environments of siblings are more similar than their social-affective environments. However, results for the Differential Peer College Orientation Scale indicate that sibings do perceive some differences in experiences likely to be related to cognitive ability. Third, the use of data on adoptive siblings and on university students and their siblings might limit the generalizability of the results. However, we found, no more than a chance number of significant differences when the results were analyzed separately for adoptive status and for college attendance.

The fourth limitation is that there was little opportunity to investigate developmental stage and sibling differential experience. The amount and type of differential experience in infancy, early childhood, adolescence, and young adulthood could differ considerably. Although the present study included both adolescents and young adults, they were asked to rate their experiences by averaging over the years when they were growing up and living at home. Hence, the analysis of developmental stage in this study was weak. The question of

within-family experiences and developmental stages is an important issue for future investigation. Finally, although the results of the study reveal differential sibling experience and indicate that these experiences are environmental in origin, our findings only begin to provide clues about the processes that mediate differential experience.

To describe differential sibling experience, we analyzed SIDE items as well as scales. Re- sults at the item level show that 65% of the sample report that their environment differs from their sibling's environment in terms of sibling interaction, peer characteristics, and events specific to the individual. The domain of parental treatment reveals fewer differ- encesm40% of the siblings perceive differential treatment by their parents. Mean scores and standard deviations for the I l SIDE scalesm Differential Sibling Antagonism, Differential Sibling Caretaking, Differential Sibling Jeal- ousy, Differential Sibling Closeness, Differen- tial Maternal Affection, Differential Maternal Control, Differential Paternal Affection, Dif- ferential Paternal Closeness, Differential Peer College Orientation, Differential Peer Delin- quency, and Differential Peer Popularity-- confirm the results based on the items. On the average, sibling experiences differ for each of the SIDE scales but to a lesser extent for differential parental treatment scales. This finding merits more thorough investigation. It would not be surprising to find that parents attempt to treat their children as equally as possible despite any existing differences. Another ex- planation might be that parental treatment is an area of experience that tends to be passively received, whereas sibling interaction and peer group characteristics are areas that allow siblings to differ actively from each other to a greater extent (Scarr & McCartney, 1983).

To examine the origins of differential sibling experience, we compared SIDE means for genetically unrelated and genetically related siblings. Overall, we found little evidence for genetic influence on differential sibling experience. Adoptive siblings show a slightly higher mean score than biological siblings for the three differential peer characteristic scales: College Orientation, Delinquency, and Popu- larity. Although the effects are small, it is in- teresting that these perceptions of differential experience are somehow mediated by genetic


differences between the siblings. It is possible that personality characteristics or cognitive abilities that are genetically influenced lead each sibling to seek out peers with certain characteristics. For example, differences in intelligence, delinquency, and sociabilitym characteristics that appear to be influenced genetically--could lead to joining peer groups that are more or less college oriented, delinquent, or popular in nature. However, after different peer groups have been chosen, differing characteristics of the groups could accentuate behavioral differences between siblings.

Constellation variables including sex, birth order, and age also were considered to be con- tributors to differential sibling experience. Al- though constellation variables have often been investigated, our study and other studies re- viewed by Dunn (1983) indicate that sex, birth order, and age of siblings tell us little about the different environments experienced by siblings within a family. In our study, for example, family constellation variables account for only 1% to 10% of differential sibling experience. The microenvironments experienced within the family are a much richer source of differential experience than are sex, birth order, or age.

The results of the present study have several implications for future research. Using the SIDE, we have shown that sibling differential experience can be systematically assessedw that it is not hopelessly idiosyncratic as had been assumed. Hence, studies of environmental variables should not ignore within-family environmental influences and might profit from looking beneath the surface of what ap- pears to be a homogeneous family environment.

Second, the fact that we have identified dimensions of sibling differential experience means that we can now move on to the main question regarding nonshared family environmental influences: Is sibling differential experience related to the marked behavioral differences that are observed between siblings? To this end, studies of younger versus older siblings, studies of siblings living at home versus away from home, studies of twins and adoptive siblings, and, most important, lon- gitudinal studies that incorporate environmental and behavioral measures on each sibling can be carried out.

A third research question involves investigating the importance of within-family environmental influences versus between-family environmental influences in understanding individual differences in development and whether the two types of influence act independently. In other words, it could be that experiencing less love as compared to a sibling is more important to an individual's development than experiencing less love as compared to a child who lives down the street.

Finally, now that we have identified systematic dimensions of sibling differential experience, it is time to begin a search for the processes that mediate differential experience within families. We examined genetic differences and family constellation variables as possible origins of sibling differential experience. These variables, however, explained only a small amount of the variance in perceived differential experience. A number of testable hypotheses concerning processes that may mediate sibling differential experience can be derived from many contemporary psychological theories, including psychoanalytic, learning, Piagetian, ethological, biopsychological, family systems, and social psychological. Sib- ling rivalry, family conflict, sibling deidentifi- cation, and split-parent identification hypotheses have emerged from psychoanalytic theory (Schachter, 1982). Learning theories of- fer sibling conditioning differences and mod- eling differences as possible processes. Piage- tian theory suggests that certain aspects of the sibling relationships (caretaking, attachment, communication, and teaching differences) are cognitively complementary rather than recip- rocal between siblings (Dunn, 1983). Etholog- ical theory points to attachment and bonding differences and sibling competition (Suomi, 1983). From a biopsychological perspective, differences in siblings' prenatal environments, as well as postnatal biological factors (illness, physical impairments, etc.), could underlie differential sibling experience. Family systems work suggests scapegoating, labeling, communication differences, and the exchange of family tetrads as possible influences on differential sibling experience (Ackerman, 1966; Minuchin, 1974). Finally, the processes of social comparison and attribution from social psychology could be considered as mediators of differential experience within the family.


O u r results indica te tha t siblings agree at least modera t e ly (.2 to .7) on thei r experiences. However, to the extent tha t they do no t agree, a t t r ibu t iona l processes may, in par t , be responsible for sibling differences in experience.

In summary , we have used the SIDE to measure several d imensions o f experience that differ be tween siblings in the same family. The 1 1 SIDE scales assess differential exper ience in four domains : sibling in terac t ion , pa ren ta l t r ea tment , peer g roup characteris t ics , and events specific to the individual . These SIDE d imens ions may serve as clues to possible ex- p lana t ions for the s t r iking personal i ty differences that have been observed between siblings and as guides in the search for processes tha t media te the differential exper ience o f siblings in the same family.

References

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Received April 24, 1984 Revision received January 21, 1985 •

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