Almost every culture on Earth frowns on the act of cheating on your partner, and yet, it's a fixture in movies, TV shows, and songs about heartbreak. It's some inarguably crappy behaviour, but why is it so pervasive? Is there any truth to the assumption that humans as a species weren't meant to be monogamous? The latest episode of AsapSCIENCE investigates the chemistry of the wandering eye, and finds that there really is a scientific basis to the saying, "Once a cheater, always a cheater."

Humans fall into the 3 percent of mammals that live their lives in monogamous pairs. We do this because, from an evolutionary stand-point, it's advantageous to pair-up and have one person hunt for resources and the other tend to the offspring. But extra-pair mating - or cheating - has become fairly commonplace in humans these days, despite the threat it poses to the all-important family unit. The culprit? Our good friend, dopamine, the neurotransmitter that helps control the brain's reward and pleasure centres.

Dopamine is released after a whole bunch of pleasurable activities, including exercising, eating, and experiencing an orgasm. Back in 2010, a US study working with 181 volunteers found that 50 percent of those with the long allele variant of the dopamine D4 receptor gene had cheated on their partner, compared to just 22 percent of those who had the short allele. On top of this, the long allele participants were also more likely to be risk-takers and susceptible to addictive behaviours, such as smoking and alcoholism.

It's not just dopamine that's at play here though: the hormone vasopressin also appears to have some kind of role. As AsapSCIENCE explains, vasopressin is similar to the 'cuddle hormone', oxytocin, because it can affect a person's capacity for trust, empathy, and social bonding. Studies done on polygamous voles have found that if you inject extra vasopressin into their brains, it increases their likelihood of becoming monogamous.

On top of that, a 2014 study done on 7,000 Finnish twins found that those who cheated had a particular variant in the gene that codes for a vasopressin receptor - one that gave them lower than average levels of the hormone. So does more vasopressin equal better at being monogamous?

It's hard to know for sure, but there are indications that for some people, being in a monogamous relationship is harder than for others, due to biological and genetic factors. And even if you happen to have none of those, be careful with how you handle your money, because that could lead you on a dangerous path too. Watch AsapSCIENCE above to find out why.