When a trait persists, other traits generally accumulate in support of it. In evolutionary theory, it’s called intraselection: If it becomes adaptive for an animal to have a heavier head (with bigger horns, for example), it becomes adaptive to also have stronger neck muscles and sturdier front legs. When our primate ancestors started to rely on fruit for vitamin C about 35 million years ago, it became advantageous for them to gain color vision to distinguish ripe fruit from unripe fruit.

Intraselection can occur within our social systems, too. A commitment to a line of work, a partner, or a belief system puts ancillary pressures on us — for example, to dress certain ways and hang out with certain people. Drug addiction is a lifestyle choice because of this kind of ancillary selection: The longer one is addicted, the more addicted friends one accumulates and the more nonaddicted friends one loses, which is why addicts generally have to abandon drug-addicted friends when trying to abandon drugs.

Mind Readers Dictionary: The Podfast : Play in Popup

Mind Readers Dictionary : Play in Popup

Intraselection is everywhere. The longer one lives in a certain neighborhood, the harder it is to up and move, because of the accumulation of collateral commitments: “We can’t possibly move now. Jimmy loves his third-grade teacher, and we’ve made so many friends.”

Ideologically, too, when we behave a certain way for long enough, we experience intraselection — incentives to embrace ideas that justify the behavior and aversions to ideas that don’t justify the behavior.

Which is why, though we may pretend we weigh ideas on their merits, we inevitably are biased in favor of ideas that confirm what we already believe over ideas that require us to shift a lot of our ideas: “Sorry, though what you say is true, I can’t possibly shift my belief. I’ve believed the opposite so long that I’ve simply accumulated too many ancillary ideological commitments.”

I’ve been divorced now for about 10 years. During my marriage, I accumulated many ancillary ideas to support my commitment. I treated marriage as success and divorce as failure, which made the shift into divorce awkward. As divorces go, ours was amicable. Gradually, I let go of my ancillary commitments to marriage and began to accumulate justifications for divorce. At first we thought it would be terrible for the kids, but, because we managed to stay kind to each other (my ex is a very kind person), the divorce doesn’t seem to be a major factor in how they turned out. Indeed, there were even certain advantages for the kids — or at least I began to count the blessings.

I’m about to remarry, and my beloved and I are planning to live in separate households for the first few years. Oddly, we’re doing it in part for the kids. We both see many advantages kids get in a divorce.

As a culture, we may have reached the tipping point on “‘Til death do us part.” With ever fewer couples lasting ’til death — or even a full decade — it might be time to admit we’re in cultural transition. I understand the reasons to resist the change. I’m personally committed to “‘Til death do us part.” But with a trend as steady as the rise of divorce, it’s also reasonable to imagine a future culture in which the persistent habit of divorce makes counting its blessings the norm. Here’s a fantasy of how it might go:

The year is 2030, and we find two mothers standing in front of a grammar school, waiting for their children.

Mom 1: And how long have you been with your husband?

Mom 2: I know it’s weird, but we’ve been together, actually, for 20 years.

Mom 1: Oh . . . I’m sorry.

Mom 2: No, it’s OK. I used to be embarrassed to admit it. Ashamed, actually. But I’m OK now. I can talk about it.

Mom 1: Well, so, how are your kids doing with it?

Mom 2: They seem to be doing OK, though I know it’s probably too early to tell. I know they’re missing out on a lot. I do worry about their logistical skills, since they’re being deprived of the opportunity to learn how to shuttle between households. My youngest loses things often. A lot of kids do, I know, and he’s still young. But . . . well, sure, sometimes I’m concerned. I know that if he had a normal childhood, he would have so much more practice. I feel pangs of guilt when I see other kids with their rollies, being passed from parent to parent. My husband and I talk about it. We’ve even talked about living in two households for the sake of the kids.

Mom 1: I think I read somewhere that diversity- development deprivation is the greater risk.

Mom 2: Well, yes, I worry about that, too. Even if we did live in separate households, our styles are so compatible, the kids would still be deprived of proper diversity training. My kids seem pretty adaptable by temperament, so maybe it won’t matter that they never got a chance to live in two household cultures, and never had to develop their chameleonic skills. We encourage sleepovers. We’ve heard that they can compensate for a failure to provide dual residence. My husband and I like a neat household, so we’ve been encouraging particularly our oldest daughter to make friends with kids with messier parents. It’s working, so far. She spends a lot of time at the Arquists’s house — a lot of sleepovers, which is good.

Mom 1: And do you feel your children are getting enough of your attention?

Mom 2: For the most part, yes. For example, we have set up a schedule like normal divorced couples, so my husband has the kids half of the week and I have them the other half. And it’s true: When parenting responsibility is scheduled, kids are less likely to fall through the cracks. We both have quality time with our kids as though we were divorced, so I say it’s going pretty well. And you? You’re divorced?

Mom 1: Yes, successfully about 10 years ago from my eldest child’s father, and then remarried twice since then. To be honest, I could have stayed in the second marriage longer, but we decided not to, for the kids’ sake.