Dec. 8, 2011 -- Having kids stresses a marriage -- but some couples manage to keep the joy in their relationship while also being great parents.
What's their secret?
"When Baby Makes Three," a report by the National Marriage Project at the University of Virginia and the Center for Marriage and Families of the Institute for American Values, finds that one-third of married couples keep their relationship strong even after parenthood.
Their recipe includes sexual satisfaction, sharing the housework and being generous. That doesn't mean you should dole out cash and lavish gifts to your spouse, according to Bradford Wilcox of the National Marriage Project.
"It's actually more regular little things, like giving her a back rub at the end of a long day. It's kind of signaling on a regular basis that you are thinking about, that you are engaged in her life," he told ABC News.
The report, based on a survey of 2,870 married people, reaffirms the conclusions of past surveys that married parents are less happy than married couples without children -- or at least they become unhappy more suddenly.
"Parents experience a significant decline after the arrival of their first child, whereas non-parents experience a more gradual decline in marital quality," the writers said.
But it found that parents -- especially wives -- are more likely to feel their life has an important purpose than non-parents. And the happiest couples are at two ends of the spectrum -- those with no kids and those with four kids or more.
One such couple, Sita and Robert Canady, both doctors from Fredericksburg, Va., have five boys, aged 11, 9, 7, 4 and 1 -- and are hoping for a girl to expand their family still further.
They told ABC News that the finding makes sense to them.
"Our commitment to each other really has deepened the more children we have," Sita Canady said.
Moms and dads who were happy in their marriage, the researchers found, were strongly committed to their relationships and had a positive attitude toward childrearing.
"Date nights" and shared religious faith also bolstered marriages, the report found.
Education is strongly associated with marital stability, according to the researchers.
As for money: Higher income does not make for happier couples, but it does make couples less inclined to divorce.
ABC News' Alice Maggin contributed to this report.