Nature 283, 375 - 377 (24 January 1980); doi:10.1038/283375a0

Data from kinships of monozygotic twins indicate maternal effects on verbal intelligence

Richard J. Rose^*, Joann A. Boughman, Linda A. Corey^†, Walter E. Nance^†, Joe C. Christian^‡ & Ke Won Kang^‡

^*Department of Psychology, Indiana University, Bloomington, Indiana 47405
^†Department of Human Genetics, Medical College of Virginia, Richmond, Virginia 23298
^‡Department of Medical Genetics, Indiana University School of Medicine, Indianapolis, Indiana 46233

Familial resemblance in intellectual skills is well documented, but its interpretation is a source of continuing controversy. The critical problem is that a family's shared genes are confounded with its shared experiences, and controls possible in animal research (selective mating, cross-fostering, and uniform or randomised environments) do not directly apply to human subjects. Conventional twin and family methods reveal substantial genetic variance in intelligence quotient (IQ) test scores, but the same methods also document significant environmental influences. Research designs which can identify the nature of these environmental factors may effect progress in the 'IQ debate' (ref. 1). The families of monozygotic (MZ) twins provide a new research design² which permits a unique assessment of maternal influences in quantitative traits. We describe here initial applications of the design to verbal IQ, with results suggesting that maternal effects significantly contribute to familial similarity in verbal intelligence.

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