Ready for a world in which a $50 DNA test can predict your odds of earning a PhD or forecast which toddler gets into a selective preschool? Robert Plomin, a behavioral geneticist, says that’s exactly what’s coming. For decades genetic researchers have sought the hereditary factors behind intelligence, with little luck. But now gene studies have finally gotten big enough—and hence powerful enough—to zero in on genetic differences linked to IQ. A year ago, no gene had ever been tied to performance on an IQ test. Since then, more than 500 have, thanks to gene studies involving more than 200,000...