Apparently, children are the joy of their parents lives. Really, they are, says a Pew Research Center survey that asked why people had kids.

And while 87% said "the joy of having children" was a key reason for having that first child, 47% said "it wasn't a decision; it just happened."

Pew listed reasons and asked how important each was: 76% cited "joy" as a "very important" reason; 11% said it was "somewhat important."

"It just happened" was cited by 35% of parents as "very important"; 12% said "somewhat important."

Pew questioned 770 parents as part of a larger phone survey of 1,003 adults. Some responses suggest major changes in society: 79% of adults know an unmarried woman who had a child; 68% know an unmarried man who did. One-third know a woman who had fertility treatments.

On the ideal number of kids, 46% of adults said two; 26% three; 9% four; 3% each said none, one or five-plus.

A third of parents with three or more children said two kids is the ideal number — but don't take that wrong, says Steven Martin of the Maryland Population Research Center at the University of Maryland, who wasn't involved in the study.

"It could be people are saying 'I'm pretty worn-out. I love them, but I would have stopped at two.' Or, they could mean 'Four was great for me, but for most families two is best,' " he says.

Among parents who said they didn't plan to have more children, "wanting to devote your time to the children you already have" was a reason cited by 76%, while "cost of raising a child" was mentioned by 72%. Of those citing finances, 48% said cost was a "very important" reason.

Another study, soon to be published online by the Maternal and Child Health Journal, appears tosupport Pew's finding that "it just happened" really does.

Almost one-quarter (23%) of 3,771 sexually active women ages 25-45 surveyed said they weren't trying to get pregnant — but they weren't not trying, either.

"We assumed 50 years after the pill, most women would be controlling their reproduction," says lead author Julia McQuillan, director of the Bureau of Sociological Research at the University of Nebraska-Lincoln.

"Women are saying things like: 'It's up to God. I don't plan these things. I just want to see what happens. I let nature decide,' " McQuillan says. "There are women who just don't want to have that purposeful approach. The data suggest they are not all very religious.

"Women can be at different points in their life. At one point, they're really trying not to get pregnant, but at another point, it would be OK."