Ever been sharing a romantic moment with a partner only for them to check out another person right in front of you?
For some couples it's harmless, for others it can be a cause for tension in the relationship.
It turns out that it's completely normal, and is something we're evolutionarily wired to do.
Jeremiah Gibson, a licensed marriage and family therapist says that being in a long-term monogamous relationship doesn't stop you from being attracted to other people.
He told Medical Daily, it's only a cause of concern if:
you and your partner have established a rule that says 'don't look at other people', or if this is done while your partner is describing something about their experience or vulnerability.
So why do we do it?
Humans are animals, and as animals our priority is to procreate, so evolution wants both men and women to seek potential partners which will maximise the chances of their survival as a race.
A recent study in Evolutionary Psychology and a 2013 study, found that women first notice a man's upper body (because they're wired to find strength), whereas men notice women's breasts, waist and hips.
The studies used eye-tracking software to map the visual activity of both men and women with different body types. Men and women both had more positive responses to women with "curves", which the study concludes is due to men being hardwired to prefer curves, as they signal fertility.
Researchers believe men have the highest chance of continuing their line by mating with as many partners as possible, which is why men are more likely to look at women after they've become sexual in monogamous relationship, according to Dr. Nancy Irwin, a therapist based in Los Angeles.
What does it mean for your relationship?
Generally, it's not a cause for concern.
If a good-looking person catches your partner's eye, generally it's just a bit of lust and while they might be looking, they don't care about the other person.
A 2009 study, published in the Journal of Experimental Psychology, showed that, when presented with photos of attractive people, those in relationships spent less time looking at the photos than single people.
One study, published last year in The Personality and Social Psychology Bulletin showed that people who don't notice attractive strangers tend to be happier and more satisfied in their relationships.
It's completely normal to be attracted to other people.
Michele Barton, a clinical health psychologist, told Medical Daily, there's nothing wrong with acknowledging beauty, but there is something wrong with disregarding our present company.