Kay Phillips1
Analyses of twin-spouse data (C. A. Reynolds, L. A. Baker, and N. L. Pedersen, 1996,Behav. Genet. 26,73-88) involved two models, called the factor/delta path model and the delta/delta path model. The results were interpreted as supporting social homogamy as the source of spouse resemblance on a measure of fluid intelligence. The authors favored the factor/delta path model based on fit to the data, but this model is not a delta path model. No delta path-based derivations can be found that lead to the set of expectations used. In particular, the delta paths shown in Fig. 2 in Reynolds, et al. yield the untenable expectation that assortment increases the phenotypic variance of those assorting; thus, the model that was actually applied to the data is not the same as that shown in their Fig. 2. The delta/delta path model avoids this problem although the use of delta paths for social homogamy was restricted to spouse pairs only. Why only spouse pairs would be affected, but not twin pairs or cotwin-spouse pairs, is unclear. The delta/delta path modelling results support phenotypic assortment rather than social homogamy, as the authors note. The present report involves reanalyses of the data and shows that there are provocative patterns in the data that were not previously modelled. The results indicate the presence of significantly greater phenotypic assortment between dizygotic twins and their spouses than between monozygotic twins and spouses. The best-fitting model also includes special twin effects that increase twin phenotypic variance relative to that of non-twin spouses. The study of differences in assortment across groups may help clarify the causes and effects of mating patterns in human populations.
Address: CDU, MDR-111, HSC, Louisville, KY 40292-0001
1Department of Pediatrics, University of Louisville Medical School, Louisville, KY 40292 2Supported by NIH Grant HD29888.