The number of American women without children has risen to an all-time high, according to a new report by the Pew Research Center. In 2008, nearly 1 in 5 women remained childless at age 40; three decades ago, that figure was 1 in 10. What's behind the shift? Here are 4 theories:

1. Americans are putting off marriage and parenthood
The research reflects a broader pattern among Americans to delay both matrimony and offspring. "People put off getting pregnant... even when they're married," says women's health expert Dr. Laura Corio of AOL Health's Medical Advisory Board. "Then they wake up one day and they're 40 and they want to have a child. It's like, hello?"

2. Women have more freedom to explore their options
About half the women who enter their 40s childless do so by choice, the study's researchers say, reflecting an overall drop in societal pressure on women to justify their existence maternally. "Women are allowed to think hard about whether they actually want children," says Jamie Kapalko in Salon, and these days they have more career and educational opportunities open to them. That gives women today "plenty of alternatives for self-definition besides motherhood." But anti-mom "prejudice" hasn't vanished: 38 percent of respondents told Pew in 2009 that an increase in childless women is bad for society; In 2007, only 29 percent of Americans felt that way.

3. Radicals have brainwashed women by devaluing motherhood
Our sexualized culture is conditioning women to see contraception and abortion as the key to freedom, says Jenn Giroux at Renew America, and children as a burden. Those who argue that women have more career options and can choose not to have children are trying to get women to buy "into the myth" that opting out of family life will "liberate them for an awesome career in a 'man's world.'"

4. Having fewer kids reduces your carbon footprint
Some women say the knowledge they're helping the environment shapes the decision, says Lisa Hymas in Grist. When we have fewer children, "our carbon footprints are lower too." It may not be their main motive, but it figures in the mix.

Sources: Wash. Post, AOL, Renew America, Salon, Grist