Cohabitation, convergence, and environmental covariances.

Temporal variation in traits has long been a central theme in epidemiology. However, human geneticists have largely avoided this topic. Recently, several authors have shown how temporal variation in relative-to-relative covariances can be accommodated within the framework of variance components analysis. The present paper attempts to clarify the mathematics implicit in their approach. A stochastic mechanism is discussed that causes covariances to converge or diverge exponentially fast as relatives cohabit or lead separate lives.