[How to reconnect with your partner after having kids]

As family psychologists such as Dr. Carl E. Pickhardt, Ph.D., point out, only children often feel like one of the adults. As with our tripartite system of government, they view the daily running of the household as a three-way power-sharing agreement. This is an issue more parents may have to deal with, now that one-child families are gaining ground. According to a Pew Research analysis of 2015 U.S. Census Bureau data, today 18 percent of mothers at the end of their childbearing years have an only child — up from 10 percent in 1976.

Tom and I have fully enabled Sylvie to feel like one of the gang, because we go almost everywhere as a trio. We’re usually too cheap to hire babysitters, and tend to travel with Sylvie, too, as she slots fairly easily into our itineraries. As a result, Sylvie has gotten used to being included, consulted, part of our in-jokes. This is not uncommon, says social psychologist Dr. Susan Newman, Ph.D., who has spent decades studying only children — a term I loathe, as it calls to mind a kid alone in a shadowy room, whispering quietly to his sock puppet “friends.” (I think we should revive the much more sprightly “oneling,” used by 19th century author John Cole in his book “Herveiana.”)

But our efforts to “empower” our oneling and make her voice heard have begun to backfire. To paraphrase Princess Diana when asked about Camilla Parker-Bowles: There are three of us in this marriage, so it’s a bit crowded.

One reason for our fluid boundaries is physical. It’s almost impossible to maintain them in a Brooklyn apartment a realtor would euphemistically call “charming and cozy,” one with bizarrely porous doors that actually seem to amplify sound. But it’s also emotional: Tom and I, like many parents of our generation, make an effort to be open and communicative with Sylvie. (“You can tell us anything, sweetheart!”)

When I was growing up, I would never have dreamed of sharing anything remotely personal with my parents. I had two siblings, and our family dynamic was solidly Us vs. Them — my sisters and I were one unit, my folks another. I wanted a different kind of relationship with our daughter.