Reincarnation, according to the Merriam-Webster dictionary, is “rebirth in new bodies or forms of life; especially: a rebirth of a soul in a new human body.” This ancient belief, a core belief of more than 800 million Hindus, has been in the news, most recently because of allegations in Andrew Morton’s new book, “Tom Cruise: An Unauthorized Biography.”

In his book, Morton says some Scientologists hoped that Tom Cruise and Katie Holmes’s gorgeous daughter, Suri, would be the reincarnation of Scientology founder L. Ron Hubbard, a man who died more than 20 years ago. The Church of Scientology denies this in a statement: “The church does not and never has believed any newborn is the reincarnation of its Founder, Mr. Hubbard-never, never, never.”

Whatever some Scientologists think of Suri’s soul, reincarnation is an increasingly mainstream belief. Madonna has said she’s a believer. So has Kate Hudson. According to a 2003 Harris poll, 40 percent of people aged 25 to 29 believed they would return to earth in a different body after they die. Popular New Age movements such as Scientology and Kabbalah teach some version of reincarnation, and best-selling books, notably by the Yale-trained psychiatrist Brian Weiss and by the therapist Carol Bowman, have brought the concept into the mainstream. Weiss and Bowman each argue that people can find happiness and peace through “regression therapy,” in which they learn about the problems faced by their former selves. (Weiss is also teaching a controversial new therapy he calls “progression therapy,” in which he helps people see their future selves as well.)

Stephen Prothero, religion professor at Boston University and a student of Hinduism, has an interesting theory about Americans’ interest in reincarnation. As life in America gets better and better, as people become more prosperous and more educated, the idea of leaving the earth forever-even with a mitigating belief in heaven-has less appeal than the idea of coming back. “We all want the here and now, and reincarnation is about the here and now,” Prothero writes in an e-mail. “Reincarnation is fueled because now people want to come back and live again.”

Reincarnation would seem to be at odds with mainstream Christianity, the majority religion in the United States. Traditionally, Christians have believed that, after death, their body and soul separate temporarily only to be reunited, at the end of time, in the general resurrection of the dead. Belief in reincarnation presents logistical-not to mention theological-problems. If souls keep cycling back to earth, which body is theirs at the resurrection? What happens to all the other bodies they’ve inhabited? Prothero argues that the popularity of reincarnation correlates to a waning of belief in physical resurrection among Christians. That’s why a third of Americans choose to be cremated these days, up from virtually none 30 years ago: they believe their souls are eternal, not their bodies. “Americans,” Prothero says, “are becoming more Hindu.”

Traditional Christians are urgently trying to reclaim Christianity from encroaching Eastern and New Age beliefs. Jeffrey Burton Russell, a Christian theologian and emeritus professor at the University of California, Santa Barbara, has spent his career trying to formulate ideas of the afterlife that jibe with Christian tradition but remain appealing to contemporary believers. These do not include, he says, either “reincarnation or an afterlife where people eat Hershey bars.” As for Suri Cruise, she’s much cuter than Hubbard-and as any parent will tell you, all children come straight from heaven.