"...First, no doubt much of the attention given to the individual is cultural. The United States is the most individualistic country in the world (Smith & Bond, 1994), and because the United States has dominated social and behavioral research, the prevalence of research concerning individuals is hardly surprising.
A second factor that has contributed to this individualistic orientation is the reliance on standard statistical methods such as analysis of variance (ANOVA) and multiple regression...
The third reason is that psychologists have dominated research in the social and behavioral sciences ..."
Very interesting book:
Kenny, D. A., D. A. Kashy, and W. L. Cook. 2006. Dyadic data analysis. New York, NY: The Guilford Press.