“…However, we realize that, insofar as factors like mother's education or father's occupation are acting as surrogates for IQ, and insofar as our test of IQ is unreliable, the evidence is equivocal. In our opinion, the inferential dilemmas these uncertainties pose for us cannot be resolved until and unless extensive longitudinal studies such as those begun by Irwin, Engle, Klein, and Yarbrough (1978) have been completed.…”