My boyfriend of a year is caring and thoughtful and perfect in nearly every way — except one. He’s really good friends with his ex. They grab lunch every week, and she’ll call him to vent or get advice. He assures me they’re just friends, but I can’t help but feel threatened. Is it crazy to tell him he needs to put some distance between them?

—Anxious

Evolution, it turns out, is a romantic doomsday prepper, setting us up with an “in case our boo disappears on us” contingency plan: basically the mating version of a reserve parachute or the Vice President.

Evolutionary psychologists Joshua Duntley and David Buss find that most of us cultivate “backup mates”: romantic Plan Bs we can plug into our life pronto if our current mate dies or ditches us or their “mate value” goes cliff diving. Though keeping a mental stash of backup boos seems like a sure sign a relationship has gone toiletward, Duntley and Buss find that even people in the happiest relationships are driven to maintain backup mates.

“Maintaining” backup mates can mean simply having them in mind. However, it can also involve efforts to keep a backup mate out of other relationships — like by sneering about the looks and “terrible” qualities of a dude they’re into (who’s actually pretty much Jake Gyllenhaal crossed with Bishop Tutu and The Rock).

Major warmfuzzy fail, sure, but it makes evolutionary sense. It’s essentially mate-loss insurance. Just as car insurance replaces your car pretty fast after you total it, having a backup mate at the ready shortens the genetically costly sexual downtime between losing or dumping a partner and slotting in their replacement.

By the way, both men and women have backup mates — three, on average — sometimes consciously, but often subconsciously: a clever little scheme by evolution. (The relationship “crimes” we don’t quite know we’re committing don’t quite leave us sick with guilt.)

Understandably, you long to tell your boyfriend to “put some distance” between himself and his ex (like by getting NASA to strap her to a rocket and blast her into space to play nuzzlylunch with the Mars Rover). However, psychologist Jack Brehm finds that telling a person what to do — trying to control their behavior — tends to be a bust, firing up a fear- and anxiety-driven freakout he calls “psychological reactance.”

The apparent threat to a person’s freedom to do as they choose jacks them into a motivational state: an intense desire to keep doing whatever they’ve been doing — often with a ferocity not seen till somebody put the squeeze on them. Additionally, activities they might be just mildly interested in tend to explode in importance the moment someone tries to take them away. (“Give me tennis or give me death!”)

In other words, telling the boyfriend he’s gotta dial it back with the ex could push him to, well, dial it forward. On the other hand, not telling him could take big bites out of you, especially if you’re “insecurely attached” (psychologists’ term for a relationship style driven by strong fears of abandonment and its feelbad cousins like anger, depression, and jealousy).

Jealousy gets a bad name, mainly from all the pain it spreads around, but it’s actually functional: an evolved alarm system, alerting us to threats to our relationships. But it also detects threats where none actually exists. Like smoke detectors, it’s calibrated to err on the side of “Better safe than charbroiled!” — especially in the insecurely attached.

That said, jealousy that seems “paranoid” might not be. Evolutionary psychologist Tom Kupfer lays out reasons some people have higher levels of jealousy: feeling their partner isn’t trustworthy, believing they aren’t as hot as their partner, and having been cheated on (in a past relationship, or, especially, in their current one!).

As for you, to determine the actual threat level and decide what to do, context matters: specifically, the nature of your relationship and the nature of theirs (that is, why your relationship exists and theirs doesn’t). First, consider that you describe your boyfriend as “caring and thoughtful and perfect in every way,” and probably not because you forgot “…and a callous dirtbag and world-class scamster.” Next, ask yourself: Is what you and your boyfriend have together rare and irreplaceable (on every level, from love to sex to fun), or…just another trolley stop on Relationship Avenue?

Finally, ask your boyfriend what he saw in his ex and why they broke up. Was there a passing issue that’s now a moot point (in which case, ruh-roh!) — or…were there “irreconcilable differences,” from emotional issues, to “we just want different things,” to big unsolvable sex problems? Best “breathe a sigh of relief!” case scenario: She’s sexually dead to him, as in, his penis is all, “I’m not getting up outta bed for that!”

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