David H Schroeder
Johnson O’Connor Research Foundation, email@example.com.
Although there have been many studies of the heritability of general intelligence (g), much less attention has been directed at specific cognitive abilities. In this study, data for specific abilities from monozygotic (MZ) and dizygotic (DZ) twins and also siblings were examined.
Samples: All of the participants in this study were clients of the Johnson O’Connor Research Foundation’s vocational-guidance program. For twins, there were 94 MZ and 261 DZ pairs and for siblings, 1,641 pairs. There were roughly equal numbers of males and females, and age ranged from 14 to 47 although most of the participants were high-school- or college-age. Effects of age were partialled from all scores.
Participants were generally upper-middle-class and college-oriented.
Measures: Participants took 15 tests of specific cognitive abilities, which yield 4 group factors and a g factor. They also took 11 tests of non-cognitive abilities including 3 tests of music-related abilities (e.g., Pitch Discrimination). Heritability estimates for the factors ranged from .85 for the Spatial Ability and g factors to .66 for Speed of Reasoning and Memory. For the specific-cognitive-ability tests, many of the values lined up with their factors—e.g., .94 and .62 for the spatial tests and .62 and .52 for the memory tests.
Interestingly, Rhythm Memory, one of the musical-ability tests, had a relatively high value, .80. The highest values for the shared-environment component were for the vocabulary test and 2 of the musical-ability tests.
The results with only same-sex DZ twins were generally similar to the results with the full DZ group. The large sample of siblings revealed a moderating effect for parents’ educational level, with higher familiarities for participants with higher levels of parental education. These results indicate a substantial genetic influence for specific cognitive abilities and group factors in addition to g.
The moderating effect for parents’ education, which is broadly consistent with previous studies of family SES, adds an important element to our understanding of genetic factors and suggests further avenues for research.