The heritability of IQ

Devlin B, Daniels M, Roeder K

1997

Pittsburgh, USA

This is a meta-analysis of 212 previous studies. The authors demonstrate that failure to consider the shared prenatal environment of twins results in an over-estimation of IQ heritability in studies of twins reared apart. When shared environment was taken into consideration the heritability of IQ appeared as much lower than in the previous evaluations and in particular those of Hernstein and Murray, the authors of The Bell Curve (Hernstein R.J., Murray C., The Bell Curve: Intelligence and class structure in American life, Free Press, New York 1994).

Many of the studies about identical twins need to be re-interpreted. Until recently researchers had not realised that identical twins can experience different intra-uterine environments even though they share the same womb at the same time. Today it appears that the main environmental influences on IQ occur early in life. See entries 0031, 0001, 0002, 0071, and 0128. Furthermore researchers do not take into account the fact that the birth of the second twin is often more difficult than the birth of the first twin.