That difference in response isn't huge, Hrdy says, but it's likely to grow as, day after day, the baby becomes accustomed to the mother responding first and begins to prefer her attention.
As biology nudges the modern mom into the traditional position of comforter-in-chief, she starts to contend with the dazzlingly high domestic standards promoted by everyone from Martha Stewart, to HGTV to your friendly neighborhood alpha mom. Today's perfectionist parent might, among other things, sign up her kid for two or three organized sports, when a generation ago one would've sufficed.
And guess who guides, supervises and chauffeurs them?
College-educated moms have increased their child care time by more than nine hours per week since the mid-1990s, according to an analysis by Garey and Valerie Ramey of the University of California at San Diego. Less educated moms are putting in an additional four hours per week.
"We get so much information and advice about everything we do, how could any mother currently feel like she's doing a good job?" says Domar.
"We need to get to the point of saying look at our mothers, our grandmothers, our great-grandmothers who didn't do anything like what we're doing for our kids now and, first of all their kids turned out OK, and second, what kind of toll is this taking on us? What kind of pressure are we putting on ourselves to excel in every way?"
Throughout our evolutionary history, mothers have been working moms, Hrdy says, gathering food and performing tasks in addition to child care.
What's different now is that the strong networks of friends and family that traditionally helped with child care have eroded, due to factors such as geographic mobility.
"Mothers don't have enough social support," Hrdy says. "That's the bottom line. The idea that mothers were ever meant to care for children by themselves in our species is not realistic."
High-quality day care could help fill the gap for some moms, she says, but progress in that area has been slow. A recent study by the Organisation for Economic Co-operation and Development found that the U.S lags behind more than a dozen countries, including Britain, Italy, Mexico and the Czech Republic, when it comes to government spending on child care.
Even reducing stress can be stressful for the modern mom.
Mathews, author of the popular Meanest Mom blog, (themeanestmom.blogspot.com), agonized for three months after being offered her dream job at a large public research institution in 2008, but ultimately decided that she couldn't have a high-pressure career and still be the kind of hands-on parent she wanted to be.
"It was arguably one of the hardest decisions I had to make, but I chose (to sacrifice some) glory and fame and future job prospects just to try to relieve some of these pressures that I had felt building," says Mathews, an assistant professor at Rollins College in Winter Park, Fla.
"Now it's kind of the happy medium where there's enough pressure to keep me motivated but not so much pressure that I'm going nuts."
If your stress level isn't manageable, you should talk to your doctor, according to womenshealth.gov, a Web resource of the U.S. Department of Health and Human Services. Your doctor can help you get counseling, suggest ways to better handle your stress and prescribe antidepressants and sleep aids.
If you're coping well but would like to do better, womenshealth.gov suggests a wide range of stress management options.
Souped-up schedules, helicopter parenting and a lack of social support combine to create a new level of pressure. And then there's the day care issue.