To Kass, the main cause of this stasis is "the successful pursuit of longer life and better health." Kass's fulminations easily lend themselves to ridicule. Nonetheless, he is in many ways correct. According to Yuji Genda, an economist at Tokyo University, the drifty lives of parasite singles are indeed a by-product of increased longevity, mainly because longer-lived seniors are holding on to their jobs. Japan, with the world's oldest population, has the highest percentage of working senior citizens of any developed nation: one out of three men over sixty-five is still on the job. Everyone in the nation, Genda says, is "tacitly aware" that the old are "blocking the door."

In a world of 200-year-olds "the rate of rise in income and status perhaps for the first hundred years of life will be almost negligible," the crusty maverick economist Kenneth Boulding argued in a prescient article from 1965. "It is the propensity of the old, rich, and powerful to die that gives the young, poor, and powerless hope." (Boulding died in 1993, opening up a position for another crusty maverick economist.)

Kass believes that "human beings, once they have attained the burdensome knowledge of good and bad, should not have access to the tree of life." Accordingly, he has proposed a straightforward way to prevent the problems of youth in a society dominated by the old: "resist the siren song of the conquest of aging and death." Senior citizens, in other words, should let nature take its course once humankind's biblical seventy-year lifespan is up. Unfortunately, this solution is self-canceling, since everyone who agrees with it is eventually eliminated. Opponents, meanwhile, live on and on. Kass, who is sixty-six, has another four years to make his case.

Increased longevity may add to marital strains. The historian Lawrence Stone was among the first to note that divorce was rare in previous centuries partly because people died so young that bad unions were often dissolved by early funerals. As people lived longer, Stone argued, divorce became "a functional substitute for death." Indeed, marriages dissolved at about the same rate in 1860 as in 1960, except that in the nineteenth century the dissolution was more often due to the death of a partner, and in the twentieth century to divorce. The corollary that children were as likely to live in households without both biological parents in 1860 as in 1960 is also true. Longer lifespans are far from the only reason for today's higher divorce rates, but the evidence seems clear that they play a role. The prospect of spending another twenty years sitting across the breakfast table from a spouse whose charm has faded must have already driven millions to divorce lawyers. Adding an extra decade or two can only exacerbate the strain.

Worse, child-rearing, a primary marital activity, will be even more difficult than it is now. For the past three decades, according to Ben J. Wattenberg, a senior fellow at the American Enterprise Institute, birth rates around the world have fallen sharply as women have taken advantage of increased opportunities for education and work outside the home. "More education, more work, lower fertility," he says. The title of Wattenberg's latest book, published in October, sums up his view of tomorrow's demographic prospects: Fewer. In his analysis, women's continuing movement outside the home will lead to a devastating population crash—the mirror image of the population boom that shaped so much of the past century. Increased longevity will only add to the downward pressure on birth rates, by making childbearing even more difficult. During their twenties, as Goldstein's quasi-adults, men and women will be unmarried and relatively poor. In their thirties and forties they will finally grow old enough to begin meaningful careers—the worst time to have children. Waiting still longer will mean entering the maelstrom of reproductive technology, which seems likely to remain expensive, alienating, and prone to complications. Thus the parental paradox: increased longevity means less time for pregnancy and child-rearing, not more.