Video: Nearly everyone has had run-ins with infidelity at some point, whether you were the guilt-ridden cheater, the devastated cheat-ee, or the unwitting (hopefully) third party in someone else's affair. (And if none of these things has ever happened to you, well, must be nice!)

Video: In the first instalment of our four-part series on infidelity, relationship and sex expert Esther Perel takes on a thorny issue: Why people in happy relationships still sometimes wind up cheating.

If you're looking for a quick and dirty exit to a romantic entanglement with someone you're not too likely to run into again, ghosting can seem like a tempting, blissfully simple option. But then, one day, you find yourself on the opposite end of the ghosting equation, lovelorn and wondering why people can't just treat each other like goddamn human beings for once!!!

Dear Lovehacker, In the third year of my marriage to my husband, I slept with his best friend. It wasn't emotional. It happened once and was a culmination of several factors (finding my worth through the male gaze, thinking that I needed to test his love for me by hurting him, several other small reasons that don't really matter in the end). I told my husband a day or two later. We stopped communicating with his friend but my husband stayed with me.

Fast forward to present day - we're on our 13th year of marriage, have a seven year-old son, and a nice, comfortable, loving life together. I can say without hesitation that we are BOTH very happy with each other. But it's recently become clear that he hasn't fully forgiven me and I'm worried he never will.

Dear Lovehacker, my wife and partner of 15 years has left me for a guy she met online in the MSN days. But she wants me to stay living in the house to fulfill her needs. She has been talking to him on and off for the entirety of our relationship and has secretly flown out to see him when I've been away.

I want to stay for my kids and give them the closest thing to a normal life as possible. And I still love her, but I guess she doesn't feel the same. Should I leave?

We’re delving into bit of a controversial topic this week. Not because we’re talking explicitly about sex. We’ve been there before. Several times.

No, this is one of those morally ambiguous areas that you know will inspire judgement and outrage from some, just for daring to humanise the subject. This is one for those who have slept with someone with a partner.

Dear Lovehacker, you might need a notepad for this bizzare love parallelogram. I am friends with two married couples. Let's call them Sally & Steve and Greg & Amelia. Greg and Amelia had been arguing recently, so Steve hung out with Amelia to console her. One thing led to another and they started banging. Amelia kicked Greg out and he worked out why. He still had the keys to their place, so he snuck in one day when they were going at it hammer and tongs. He snapped some pictures and sent them to Sally. And then things got even worse.

Here's another reason to hate bankers and stock brokers: according to a poll by adulterous dating site Victoria Milan, the finance industry is the most unfaithful profession. So in addition to routinely destroying the global economy in an endless cycle of greed and damage control, they're also more likely to cheat on their partners. Tch. Here are the rest of the top ten.

Most people believe that cheating in a relationship is bad. At the same time, it's common enough to be a trope in virtually every romantic comedy you've ever seen. This video explains some of the reasons why this dichotomy exists.

It’s a time honoured tradition that the banker always steals from the bank in Monopoly. Most cheating in board games isn’t anywhere near as brazen. At least, not if you want it to work. Being tricky is tricky. Cheat too often or too heavily and nobody will play with you. Cheat a little and your opponents may not even notice.

You went and did the unthinkable: you cheated. Now what? If your partner decides to end the relationship, there's not much for you to do, but if they want to stay together, you have some work ahead of you.

Knowing about the infidelity of a friend's significant other can be the most awkward situation ever. You want to inform and console your friend, but you don't want to damage your relationship with them and end up being the dead messenger. If you think (or know) your friend is being cheated on, here are some things to consider.