Washington ? Presidential candidates Fred Thompson, John Edwards and Joe Biden belong to a club that no one wants to join. Each has lost a child.

In their campaigns, they raise the subject of their personal tragedies only occasionally and, even then, usually in a tangential way.

Edwards has talked about the death of his 16-year-old son in the context of his wife’s battle with cancer. Biden has recalled the people who helped him when his wife and young daughter were killed in an auto accident. Thompson has cited his 38-year-old daughter’s death as one of the events that have shaped his outlook.

“It’s a sensitive subject,” said Stanley Renshon, a political science professor and psychoanalyst at City University of New York. “The loss of a child ordinarily is devastating to families. It ranks up there with loss of a spouse, probably even higher on the life stress rating scales they use in psychology.”

For the candidates, there are risks and gains to opening up to voters on such sensitive subjects, experts on political psychology said.

“Some (voters) consider these matters private. They wouldn’t talk about it themselves. It makes them uncomfortable,” said Jerrold Post, director of the political psychology program at The George Washington University in Washington.

“Yet I think on balance there is a humanizing aspect to it that they, too, have known what it’s like to lose a child, a loved one,” said Post, a psychiatrist and a former psychological profiler for the CIA.

Thompson, the actor and former GOP senator from Tennessee, recently mentioned his daughter’s death from an accidental overdose of prescription drugs as he sought to explain a lifetime of events that prompted him to consider running for the White House.

It is not unusual for tragedies such as the loss of a child to serve as a source of motivation and resolve, Post said.

“In particular,” Post said, “what one sometimes sees – and there are no total rules in this by any means – is that with the emphasis on the fragility of life there comes a kind of commitment to make every moment count and to focus all the more closely on the importance of relationships and how one lives one’s life.”

In March, Edwards and his wife, Elizabeth, announced that her breast cancer had spread to her bone. As an example of their ability to cope with adversity, the couple cited the death of their son, Wade, in a car accident in 1996. Wade was driving to the beach in North Carolina when a strong wind blew his Jeep off the road and it flipped over.

Biden was 29 when he was elected to the Senate in an upset in 1972. Before he could take office, his wife, Neilia, and 18-month-old daughter, Naomi, were killed when a tractor-trailer truck hit the family’s station wagon.