Twin-singleton differences in intelligence?
The twin method has been criticised for its alleged non-generalisability. When population parameters of intellectual abilities are estimated from a twin sample, critics point to the twin-singleton differences in intrauterine and family environments. These differences are suggested to lead to suboptimal cognitive development in twins. Although previous studies have reported twin-singleton differences in intelligence, these studies had two major drawbacks: they tested young twins, and twins were compared with (genetically) unrelated singletons. To test accurately whether twin-singleton differences in intelligence exist, a group of adult twins and their non-twin siblings were administered the Dutch WAIS-III. The group was large enough to detect twin-singleton differences of magnitudes reported in earlier investigations. The data were analysed using maximum likelihood model fitting. No evidence of differences between adult twins and their non-twin siblings on cognitive performance was found. It is concluded that twin studies provide reliable estimates of heritabilities of intellectual abilities which can be generalised to the singleton population.
Posthuma
D
D
Department of Biological Psychology, Vrije Universiteit Amsterdam, The Netherlands. danielle@psy.vu.nl
De Geus
E J
EJ
Bleichrodt
N
N
Boomsma
D I
DI
D000328:Adult; D000704:Analysis of Variance; D016009:Chi-Square Distribution; D003071:Cognition / Q000502:physiology; D005190:Family; D005260:Female; D006801:Humans; D007360:Intelligence / Q000235:genetics* / Q000502:physiology; D007361:Intelligence Tests; D016013:Likelihood Functions; D016014:Linear Models; D008297:Male; D009426:Netherlands; D009679:Nuclear Family / Q000523:psychology; D012931:Social Environment; D014429:Twins, Dizygotic / Q000523:psychology; D014430:Twins, Monozygotic / Q000523:psychology