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The Well Column Tara Parker-Pope on living well.

During a late-night monologue, the talk show host David Letterman once asked a series of questions about gay marriage.

“Who gets the bachelor party?” the comedian wondered. “Who goes downstairs in the middle of the night to check on the noise? Who forgets the anniversary? Who refuses to stop and ask for directions? And which one of you will take forever to get ready?”

The joke generated the expected laughs from the audience, but it was also a telling example of the effort to define same-sex marriage by comparing it with heterosexual marriage. While there are many similarities between gay and straight marriages, a decade of social science research shows that same-sex couples have also adapted the institution in various ways.

Gay relationships tend to be more egalitarian, in part because same-sex couples don’t divide work along traditional gender lines. Gay couples also report less conflict and more happiness in their relationships. And because gay couples often lack the support of family members, they tend to receive social support from an extended network of friends.

Now that two Supreme Court rulings have paved the way for more same-sex couples in the United States to marry, relationship researchers say there are important lessons to be learned by continuing to study both successful and unsuccessful gay marriages.

Recently, the National Institutes of Health approved a $1 million study of gay and straight couples who have been tracked for 10 years by researchers at San Diego State University. One of the largest studies of its kind, the research began after Vermont legalized same-sex civil unions in 2000. The original study focused on nearly 1,000 couples, including same-sex couples and their heterosexual married siblings. The inclusion of siblings allowed researchers to compare similarities and differences between gay couples and heterosexual couples of similar ages, family and religious backgrounds.

Today, about 750 couples remain in the cohort, and the new N.I.H. study will be a 10-year follow-up to determine how the couples have fared.

“The same-sex couples who got civil unions in Vermont in 2000 will always be the longest legal gay couples in North America,” said Esther Rothblum, a professor of women’s studies at San Diego State University and an author of the research. “There is so much to learn by following them, but we really know very little. Most of the questions people ask me about same sex marriage, my answer is, ‘We don’t know yet.’ ”

After the couples had been followed for three years, the first data from the research were published in 2004 in The Journal of Family Psychology, with a follow-up in 2008 in the journal Developmental Psychology. While the findings were illuminating, much of the research raised more questions about how gender can influence simple marital dynamics.

One of the most notable findings was that by nearly every measure, same-sex couples reported higher levels of happiness in their relationships than straight couples. Gay couples reported far less conflict than heterosexual couples and higher levels of intimacy. Gay couples were more likely to feel that they could confide in their partners, experience high levels of affection and be happier with their sex lives.

One reason same-sex couples may report more satisfaction and less conflict is that their relationships tend to be less defined by traditional roles. Men, gay or straight, often find it easier to communicate and share perspective with other men. Women typically find communication easier with other women.

“What I like to say is that if men are from Mars and women are from Venus, then it helps to have two Martians negotiating a conflict,” Dr. Rothblum said. “All things being equal, with gay couples you have two beings who have been socialized in similar ways.”

Even though gay couples reported more overall happiness in their relationships, they also were more likely to break up. After three years of follow-up, the breakup rate among gay couples in the study who were not in civil unions was 9.3 percent. Gay couples in civil unions had a breakup rate of 3.8 percent, while only 2.7 percent of heterosexual couples had split up.

While that may seem like a contradiction, the finding suggests that external factors — like family pressure, children and financial commitments such as a mortgage — may play a greater role than simple happiness in the duration of heterosexual marriages. Gay couples typically report fewer close family relationships and are less likely to have children.

The data also indicate that legal recognition, whether it’s a civil union or a marriage, is a sort of glue that can help couples, gay or straight, sustain a long-term relationship.

“When things aren’t going well, same-sex couples may find it easier to just split up,” Dr. Rothblum said. “Now that the marriage laws are becoming more widespread, gay couples will also have to go through the legal process of breaking up.”

Another important lesson from the research, she noted, is that marital duration is not a good barometer of marital happiness.

“Often when we talk about breaking up or divorce or unhappiness, we assume those are all the same things,” Dr. Rothblum said. “But presumably there are lots of people still married who are miserable and others who are quicker to break up. Just because heterosexual couples stay together longer does not mean they are always happy. It may just be that there are external reasons, more societal pressures that couples face to stay together.”