Table 1. Explained variance in mean intelligence test scores among the birth cohorts of 1940–1980.

Parameter	Cohort-level associations		Individual-level associations
	r	R2	r	R2
Family size	-0.75	0.726	-0.06	0.014
Height	0.97	0.990	0.26	0.067
Education	0.99	0.994	0.66	0.487