latent growth model for phenotypic data, found both slopes and intercepts to be heritable in the age range from 17 to 23. This result is consistent with those presented here. A genotypic effect beginning at age 17 and continuing past age 23 would produce a difference in slope attributable to a difference in genotype. An example of this is observable in panels 2 and 4 of Figure 6, where fitting a slope-intercept model to the separate genotypes in males over the age range of 17 to 23 would produce lines with different intercepts and different slopes. This type of genotypic effect would also be consistent with the first sentence of the conclusion, “... it appears that the genes influencing slope are largely, if not entirely, the same ones influencing intercept …” (Carlson and Iacono, 2006, p. 478). However, the heritability of slopes might be the result of age invariant genetic effects, and does not have as a consequence the age variation of genetic effects.
