DEVEL OPMENT A N D N EUROBIOLO G Y As sist an t Ed itor: P aul j. Lombro so , M .D . Genetics o f Childhood Disorders: III. Genetics an d Intelligence ROBERT PL OMIN , PH .D . Gen etic resear ch h as mad e imp ortant di scoveries ab ou t intell igence durin g the p ast fe w de cades. T o outlin e s om e of these find ings, I w on't s pend s pace on the me asur ement of i ntelligen ce ex c ep t to s ay that what I m ean b y intelligen ce is g eneral cognitive abili ty d ef ined a s g . All rel iabl e and va lid t ests o f cog nitive a bility i ntercorrelare a ta m odest l evel -g i s w hat the y h ave in c om m on . g is o ften ass essed a s a to tal s core ac ross d ivers e c ognitive te sts as i n i ntelligence (IQ) te sts, although i t is more ac curately ind exed b y an unrotated prin cipal co mponent th at b est refl ectsw ha t i s i n co mmon a mong th e te sts. Ne ar lya ll g enetic dat a have b een o btained usin g me asures d eveloped from th is p sychometric p erspective, prim arily IQ tests . On e new direction for g enetic r esearch on i ntelligence i s to in vestigate o t he r m easures s uch as in f ormation-processinga nd m ore direc t mea sures o f b rain fun ction s uch a s e voked p otentials , posit ron em ission tomograph ic s cans, and f un ct i onal m ag- net ic re sonance imag ing a nd to explain h ow th ese me asures r elate t o g . g cl early run s i n f amilies . Th e correl at ions f or fir st-degree rel atives living together aver age 0 .43 for more th an 8,0 00 paren t -offsp ring p airs and 0 .47 for more th an 2 5,000 p airs o f s iblings. Ho w ever, g m ig ht r un i n fam ilies f o r re a son s o f nu rture o r of n ature . In stud ies in volving mo r e th an 10 ,000 p airs of tw ins, the aver age g c orrelations a re 0.8 5 for id entical twin s and 0.60 f or s ame-sex fr aternal tw ins. Th ese tw in dat a su ggest a g enet ic eff e ct si ze ( heritability) that e xplains about h alf o f th e tot al v ariance in g sc ores . Adopt ion s tudies a lso yi eld est imat es o f substant ial her as si milar f or g as id en t ical tw ins r eared togeth er. A doption s tudies o f o ther fir st-degree rel atives a lso i ndicate s ubstantial h erita b ility, a s ill ustrated below b y re c en t re sults from the Biolog ical Adoptive - - - - 0 .4 Control _ , - - ........ <, ......... ................ <, . ......-: .z-> ........ \ \ \ --- .... " / _ ,.t. . " / ... __ / 0 .3 0 .2 0 .1 m c: o o U C l c: 'C 0- m :t : o C l!? 8!. "0 Q) Ol Q) -Q .1 0 2 4 6 8 10 12 14 16 1 8 Age of Child F ig . 1 P arcllt - o tl s pring cor rela tions b etween pa r e nts" fir .'t p rin c ipa l co mponent s co re s a nd c hildren 's fi rst p rincipal compo ncl1l sco res der i ve df ro m t e sts of spec ifi c cogn itive a bilitie s fo r a doptive . b iologic al. an d co ntro l p arents a nd t h eir c hildren a t J. 4 . 7 . I). 10 , 12 . 1 4 . a nd 1 6 years . T he N 's rang e fro m 2 7 to 42 for b iol o gical fat hers, 123 t o 1 95 fo r bi ol ogical m other s. 1 21 to 1 94 for a doptive p arents. an d 11 5 to 21 6 for cont rol pare nts. Ada pted f rom Plorn in R .F ulker DW . Co r le y R . De fri e s JC ( 1997 ). Natu re. nu r t u re a nd cog niti ve de ve lopm ent fro m I to 1 6 y ears: a p a rcur-o ff spri ng a dop tion s tu dy. Ps ycho! Sci 8 :4 42-44 7 ( h y p ermis s ion of pub lish er) . 7 86 J . AM . A CAD. CHIL D A D OL ES C. P SY CHIA TR Y, .18 :6 , J U N E I' ) ')')
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