It is one of the more unusual battle lines in the culture wars.
Liberals, it is said, have a baby problem. They don't have enough of them, compared to conservatives. And this failure to replenish their ranks is a reason why they lose elections. Call it a fertility gap.
"The political right is having a lot more kids than the political left," Syracuse University professor Arthur Brooks says. "The gap is actually 41 percent."
Studying numbers from the General Social Survey -- a government survey of social trends -- Brooks found that 100 unrelated liberal adults have 147 children, while 100 unrelated conservatives have 208 kids.
That makes a difference, Brooks says, because "80 percent of people that express a political party preference are voting like their folks."
Hence, more Republicans.