Some guys are jealous of the simplest things—a text message from a male co-worker, you spending more time with your boss, or even an old flame from high school visiting town. However, if there’s one thing that most men will truly get upset about, it’s you having sex with someone else, no strings attached. According to researchers David Frederick of Chapman University and Melissa Fales of UCLA, majority of men (54 percent) admit that they’ll get more jealous if their partners have sex—just sex—with someone else as opposed to if they get emotionally involved (regardless of whether they had sex or not).

Find this behavior strange? Researchers have a few theories about this behavior.

Baby crazy
“Before DNA testing, a man could never be sure if a child he was helping a woman raise was his own child,” reports Elite Daily. This means that if a woman had sex with someone else and then had a baby, it wouldn’t make sense, evolution-wise, to stay in the relationship.

Sexual insecurity
Another theory is that if a woman went to someone else for sex, that might mean her guy has poor bedroom skills—an obvious sore point for many men out there. On the flipside, a woman might be more jealous of her man developing emotional connections because it questions her “core feminine ability to bond.”

Is your partner the jealous type? Let us know what he’s usually upset about in the comments below.

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