As much as we might hate to admit it, we’ve all gotten a little jealous around other women. Sometimes you see another chick talking to your partner, and you just can’t help but turn green with envy. But then there are times when it doesn’t phase you at all. What’s the deal?

According to a new study published in in the journal Biology Letters, your jealous behavior might have more to do with your hormones than your insecurities.

For the study, researchers showed 220 women two photos of a woman. One of the photos was taken at a time when she was ovulating, and the other photo was taken while she wasn’t ovulating. Here’s an example (the photo on the left is during ovulation):

Image courtesy of Biology Letters

Just from looking at the photos alone, the researchers found that the study participants who were at peak estrogen levels in their monthly cycles (which happens every month just before you’re about to start ovulating) were more likely to think the ovulating women looked like date-stealers and to perceive them as a threat. That’s one heck of a powerful photo.

The culprit behind our judgments: all that estrogen running through our veins. The researchers only found this effect when the woman looking at the photos was at that high hormonal point in her cycle.

The researchers say that the link between our hormones and our primal response to who we view as a threat all comes down to competition. Our inner cavewoman does not like the idea of another woman hanging around when we are biologically prepped to reproduce.

Still, there a few caveats: The study was only done with a small group—and the researchers note that they’d need to be able to replicate their findings on a larger scale before we can fully rely on this data. And although the study authors write that women tend to look more “attractive” during ovulation, the two photos look almost identical, so there’s a chance this could all be coincidence. Plus, attractiveness can be pretty subjective, don’t you think?

That said, the findings are interesting. The next time you feel the claws about to come out, think about where you are in your cycle—it may just be another annoying thing you can chalk up to hormones.