The new study, a working paper published in September, used the same technique, but with fertility data from 2008 to 2013. “We were surprised to find that it was not true anymore that having a girl encouraged additional births,” said Francine Blau, an economist at Cornell and one of the paper’s authors. “There could be a daughter preference.”

Ms. Blau and her colleagues said the new data shows that other factors now outweigh the preference for sons. That could indicate a preference for daughters, or it could be a combination of things.

In general, Americans — especially men — have been more likely to say they want a child of their own gender. In the 2011 Gallup survey, 31 percent of women wanted a boy and 33 percent a girl, while 49 percent of men wanted a boy and 22 percent a girl.

Part of the reason is parents want to share interests and hobbies with a child, research shows, and think this will be based on gender. Now that girls play sports and do other things that used to be considered masculine, fathers might feel more of an affinity for them. Stereotypes about what boys spend their time doing have not changed as much.

As women have gained more decision-making power in marriages, and become more likely to be single mothers, they might be exercising their daughter preference more often than they used to. That could explain the difference between the Gallup survey responses and the results from the new research. (Gallup said it was planning another survey on this question this year.)

“It could be men are just as biased as they always were, but have less of a say in having another kid,” said Mr. Dahl, an economics professor at the University of California, San Diego.

There could be explanations other than a preference for daughters. For example, people might stop having children after having daughters because daughters cost more than they used to since they are more likely to go to college, researchers said, though it’s unclear if this has affected people’s family planning decisions.