Walnut Creek, Calif. ? There may be some truth to the idea of a “seven-year itch.”

Marriages that end in divorce typically last seven to eight years, a new study reveals. Most people about 75 percent of those who divorce will remarry. But even second marriages that self-destruct typically do so after about seven years.

The Census Bureau on Thursday released its first comprehensive portrait of marriage and divorce in the United States in a decade.

The report, based on 1996 statistics, predicts that 90 percent of Americans will say “I do” at some point during their lifetime.

But the study also confirms a widely touted and discouraging statistic: Nearly half of all first marriages will end in divorce.

Marriage therapists offer a variety of explanations for the problems that can develop seven to eight years beyond the wedding vows.

“People have one child and then they have a second child that’s pretty disruptive to relationships,” said Peggy Thompson of Collaborative Divorce Associates in Orinda, Calif.

Rather than viewing it as a seven-year-itch, Thompson considers it a growth period in which partners often change.

“Their interest changes, their focus changes,” she said.

John James, a marriage counselor based in Lafayette, said a power struggle can develop several years into a marriage.

“It may be that after that many years people are settled into the routine stage,” James said. “They know a lot about each other, so they’re settled into some pretty strong habits. That may be where they start to get into a conflict over those habits.”

People wait longer to get married these days, but they are also more likely to opt for divorce than previous generations.

By the time they reached age 40, only 15 percent of men and women born from 1925 to 1934 were divorced. These are the parents of many of today’s baby boomers.

By contrast, 31 percent of people born between 1945 and 1954 were divorced by the time they reached their 40th birthday.

Women generally fare worse than men economically after a divorce, the study reveals.

The study found that 21 percent of recently divorced women had incomes below poverty level, compared to 9 percent of men.

College education can also play a role. People with higher education are more likely to marry and less likely to separate, the study found.