“Don’t worry, it will get better.” This is the message that veteran parents usually offer bleary-eyed mothers of infants struggling to keep it together. But what if it gets worse?

This is one possible message of a new study by Arizona State University researchers Suniya Luthar and Lucia Ciciolla published in the January issue of the journal Developmental Psychology. The authors compared various measures of maternal happiness by the age of the child and found that it was not mothers with infants who were the most miserable. It was the ones with middle schoolers.

These mothers scored highest on measures of stress, loneliness, and emptiness, and they also reported the lowest levels of satisfaction and fulfillment. So what is going on here? Well, hormones, for one thing. As the onset of puberty has come earlier and earlier, mothers have to deal with the consequences when their kids are age 10 or 11 instead of 13 or 14.

But more importantly, middle school is a time when children are becoming independent—or at least they’re trying to. It’s getting very hard in our era of hothouse parenting to figure out how and when to let go of some of our control over kids. Middle school is the age at which total control starts to become untenable. Yes, it’s ridiculous to hold your 10-year-old’s hand when she crosses the street. No, really, nothing will happen if you let your 11-year-old son walk home from school.

Middle school is also a time when the peer group’s power grows exponentially. Parents of middle schoolers are still vying to be the primary influence over their children, but they have a lot of competition. In his new book, The Collapse of Parenting, Leonard Sax argues that the failure of mothers and fathers to counteract this peer influence is causing children today to be more entitled and less respectful than previous generations. This probably also adds to parental stress.

The authors of the new study, though, also consider that “the developmental trends we documented partly arise from challenges that mothers themselves experience, as they are transitioning to midlife.” As mothers are having kids later in life, the midlife crisis may well overlap with children’s preadolescence. But the midlife crisis may also be intimately related to children’s sudden push for independence. If your life has, for more than a decade, revolved around planning and protecting every aspect of a child’s experience, the first years of independence are going to cause problems for your sense of purpose and identity.

Finally, the authors suggest that this problem of middle-school mom unhappiness may be particularly pronounced among highly educated and affluent mothers, who were over-represented in their survey sample. This, too, implies that it is not just the fact that our kids are becoming more annoying when they hit middle school. The authors believe that some of the stress mothers are experiencing could be the result of their desire to get their kids into college: “In upwardly mobile settings, the pressures for ‘resume-building’ have increasingly encroached into junior high schools.”

All of which suggests that we have only ourselves to blame for this unhappiness. While the trials of living with babies are natural and unlikely to change—sleeplessness, frustration with beings who can’t communicate effectively, a change in relationship with one’s spouse—the problems of living with a middle schooler are artificially created. Maybe it will get better. But probably not soon.