The nones' retention problem

Michael Gerson offers more statistics about Americans' declining attachment to institutional religion.  One statistic had escaped my attention previously:

According to Pew, 74 percent of the nones grew up in a religious tradition of some sort. Yet while conversion has increased the ranks of the nones, retention is not particularly good. Protestantism, for example, loses about 20 percent of those raised Protestants. Of those raised unaffiliated, 40 percent fall away from the non-faith and rebel toward religion, making for a new generation of awkward Thanksgivings.