Ambivalence Actually

My boyfriend of two years read my diary and found out that I had expressed feelings for another guy while we were together. I never acted on them (and I wouldn't have), and I probably shouldn't have told the guy I liked him. But my boyfriend shouldn't have been reading my diary! He broke up with me, saying he wouldn't be able to forgive me. Now he wants to come back. What should I do? I don't feel that I can trust him now.

--Disturbed

Having regular sex with you does not give another person the right to rake through your diary like it's the $1 bin at Goodwill. Your boyfriend probably equated your approaching this other guy with an attempt to cheat, but it sounds like it was something different -- a sort of preliminary investigation into whether you had any chance with that guy. It turns out that we have a sort of inner auditing department that gets triggered to calculate whether "the one!!!" should maybe be that other one. Accordingly, research by evolutionary psychologists Joshua Duntley and David Buss and their colleagues suggests that we evolved to cultivate romantic understudies -- backup mates whom we can quickly slot in as partners if our partner, say, dies or ditches us or their "mate value" suddenly takes a dive. What else might trigger going for -- or at least testing the waters with -- a backup mate? Well, though you didn't have sex with this other guy, it seems instructive to look at why women tend to have affairs. Research by the late psychologist Shirley Glass finds that women view seeking love and emotional intimacy as the most compelling justification for cheating. (Seventy-seven percent of women surveyed saw this as a compelling reason to have an affair, compared with only 43 percent of the men. Men were more likely to see sexual excitement as a compelling justification to stray -- with 75 percent of the men, versus 53 percent of the women, giving that reason.) As for whether you should take your boyfriend back, the question is: What was missing that led you to try to trade up, and is it still missing? We're prone (per what's called the "sunk cost fallacy") to want to keep putting time and energy into things we've already put time and energy into, but the way to judge whether something's actually worthwhile is to assess how well it's likely to pay off in the future. If you feel (and act) more certain about your partner, he is less likely to have mate-guarding impulses triggered (like the temptation to snoop). However, if you do get back together with this guy, privacy rules need to be spelled out -- and followed. (Presumably, your daily journal entries start with "Dear Diary," not "To Whom It May Concern.")