33. Fulker, D. W. & Cardon, L. R. (1992) What can twins studies tell us about the structure and correlates of cognitive abilities? In T. Bouchard & P. Propping (Eds.), What are the mechanisms mediating the genetic and environmental determinants of behavior? Twins as a tool of behavioral genetics (pp 33-52). Chichester, England: John Wiley & Sons.

This paper discusses the strengths of the twin design in investigating the biological and social significance of cognitive abilities. An evolutionary perspective on IQ is also presented. How differences between individuals at the DNA level can be shown to be a cause of individual differences in IQ or any phenotype, by use of the twin design is discussed. Next, a discussion of multivariate twin analysis is presented, along with a brief history of the field with respect to specific cognitive abilities. The weaknesses of the older studies and the strengths of the current structural modelling approach are discussed. State of the art multivariate longitudinal analyses of our CAP and twin data are then presented. A longitudinal analysis of IQ yielded the conclusion that there is continuity of time-specific genetic influences from 1 through 7 years, with much new genetic variation appearing at year 7, after the child experiences the first year of schooling. There was also a small but constant influence of the family environment on continuity, while the unique environment only contributed to change across time. Next, a hierarchical analysis of specific cognitive abilities at year 7 indicated a strong unique environmental higher-order common factor, with unique environmental factors for spatial and memory abilities only. Shared environmental influences could not be detected. Genetic factors common to each of four specific cognitive abilities, in addition to genetic g factor of general intelligence were found necessary. Next, a model combining the hierarchical model of cognitive abilities at each age with the age-to-age transmission model is discussed. Finally, the integration of quantitative genetic analyses of this type with information on molecular RFLP markers to detect specific genes causing IQ is introduced.