As measures of intelligence, we examined both years of education (
EduYears) and IQ. We designated
EduYears as our primary measure because the attainable GWAS sample size for
EduYears is currently larger than that for IQ. Previous studies have found a high genetic correlation (∼0.7) between IQ and
EduYears, and the top SNPs identified in the respective GWAS of these traits show very strong concordance. Our two measures of brain size are adult intracranial volume (ICV) (
Hibar et al., 2015) and infant head circumference (HC) (
Taal et al., 2012). ICV is an excellent proxy for brain volume with the advantage that it does not decline sharply with age (
Davis & Wright, 1977); thus, any correlation with intelligence cannot be explained by the effects of neural atrophy on cognitive decline. According to bivariate LD Score regression, adult ICV and infant HC show a genetic correlation of 0.75