Dear Carolyn:

My girlfriend has told me about her unfaithful past. It has me worried quite a bit. She told me she dated someone for three years, and in the last year she became unfaithful. It started with just hanging out with a guy she liked, but she didn’t engage in anything physical. However, she did meet someone at work and that went into physical territory — weekends away to hotels. Her then-boyfriend found out and she lied to him.

After that ended, a year and a half ago, she became very promiscuous. We met four months ago and have been dating ever since. She told me she would never cheat on me, and that her past was full of mistakes. She says she used sex to cope.

Sometimes I worry. I’ve heard the saying, “Once a cheater, always a cheater,” and I’m trying my best not to become a jealous, possessive boyfriend. Any advice?

R.

(Nick Galifianakis/For The Washington Post)

How come you never hear, “Once a freeloader, always a freeloader,” or, “Once a whiner, always a whiner,” or, “Once a self-obsessed, chaos-sowing pain in the butt, always a self-obsessed, chaos-sowing pain in the butt”?

People can be a bad relationship bet for countless reasons, often in glorious combination. Fixating on only one of them automatically forces you into a narrow view of a person, just when thinking broadly would optimize your judgment — not just of her, but of yourself with her.

For example, why are you stressed by her cheating but not relieved by her honesty? Is cheating the only, or even the most serious, way she could hurt you? Have you ever mistreated a girlfriend? Did you disclose exactly how, when and why to your current girlfriend?

Of course it makes sense to be concerned about her destructive past. Of course it’s going to be on your mind. Of course infidelity is near the top of the bad-news food chain.

But tease out the contributing elements of cheating, and you’ll get a much less tidy, but much more useful set of traits to screen for — in part because they’re behind all kinds of destructive behavior, not just infidelity.

There’s dishonesty at work, for one. There’s selfishness. There’s thrill-seeking. There’s often impulsiveness, or susceptibility to immediate gratification. There’s pain relief. There are, sometimes: cruelty, vanity, greed, cowardice, depression, addiction, denial, contempt.

Being possessive of someone, or employing the proverbial short leash, does nothing to offset any of these destructive tendencies. What does keep them from infecting a relationship is mastery of them by your girlfriend, and a certain amount of acceptance of them by you both.

The former is pretty straightforward. If she has looked the worst of herself in the face, identified her errors and frailties, worked to overcome them, and accepted that some element of this work will be lifelong, then I like her chances in a relationship better than I do some people’s who’ve never once colored outside a line.

As for the latter: I like both of your chances if you’re both mature enough to recognize there’s no such thing as trusting someone never to do you harm. There’s only trust that it won’t be purposeful or negligent — and that you’re prepared to handle it. Even that trust isn’t foolproof, but I don’t advise committing to someone without it.