Higher divorce rates are one result of Americans' longevity, says a researcher who claims many marriages cannot stand an extra two decades until "death do us part." "Longevity has added as much as a quarter of a century of close living to many marriages," according to Dr. Holger Stub, a researcher in the field of social gerontology at Temple University. "This alone exposes marriage to a phenomenal increase in disruptive influences and culminates in high divorce and separation rates." Stub, who spent a year studying the consequences of longevity, said even though marriages may now have a potential length of 50 years, many won't last half that long. "Like the policeman, mailman, secretary or soldier who retires from one career and takes up another, increased longevity allows men and women virtually to bargain for two marriages, one for the young family years and another for the postparental period," Stub said. "Though this sounds amoral and cynical to romantics or the religiously oriented, it is not merely a possibility but is actually taking place." Stub said a "relatively large minority" of husbands and wives separate at middle age, after their children have grown and left home, and remarry "by plan and design." He said the prospect of a long life "changes the whole perspective" of what people feel they can do with the rest of their lives. "The increased length of life allows new kinds of steps, alternatives and decisions for human beings," Stub said.