Some have suggested that differing pre-adoption experiences, including age at adoption, explain the racial patterns in the results. Lee (2009) argues against this interpretation, pointing out that there is no evidence from other studies that variables such as age at adoption exert an effect on IQ lasting until late adolescence. In the Minnesota study, the proportion of IQ variance associated with pre-adoption variables declined from .32 to .13 between ages 7 and 17. Lee further suggests that causality may run from IQ and other behavioral variables to differences in pre-adoption experiences rather than the other way around, and that race by itself as a visible characteristic may have affected pre-adoption experience.