Heritability of various aspects of cognitive development is very high (e.g., around 70 percent for IQ) in kids from high-socioeconomic status (SES) families but is only around 10 percent in low-SES kids. Thus, higher SES allows the full range of genetic influences on cognition to flourish, whereas lower-SES settings restrict them. In other words, genes are nearly irrelevant to cognitive development if you're growing up in awful poverty--poverty's adverse effects trump the genetics.