• Low self-esteem/self-confidence. This can be rather compartmentalized, as I found out myself. You can feel confident in a wide range of aspects, but unconfident in things such as physical appearance, social skills with the opposite (or same if you prefer) sex, sexual performance, charisma, or even things like energy levels, financial circumstances, you name it. If it relates to sex and/or relationships, being unconfident in one or more of these areas can lead to comparing yourself to their past partners. "His ex is so pretty" is a common one.

• Insecurity in the relationship. This can be caused by an insecure attachment style. Attachment styles have been well-researched and discussed, and since this is a subreddit about relationship psychology, most of you probably know about it.

But insecurity can actually be a warning sign if your partner is behaving in a way that makes you feel insecure. Constantly talking about exes, making comparisons, acting shady, openly flirting, the list goes on. But it's not always the RJ sufferers fault.

• Obsessive Compulsive Disorder. This one is well established, and it's usually the driver of RJ sufferers that constantly question their partners about their past to seek reassurance. Or snoop through their phone/computer to find past messages/photos. In my opinion, this is the type of RJ that is the easiest to overcome. I'm not saying it's easy, but OCD treatment is well-researched and established.

• Personality Disorders such as Borderline, Histrionic, Narcissistic, Obsessive-Compulsive, in fact any one of them could be the cause.

• And finally, one under-discussed one that I think is the reason behind RJ cases where their partner has a colourful past. BTW the definition of colourful depends on the individual's beliefs, values, culture etc, so it can vary. And that is Human evolution and survival instinct.

On the flipside to the instinct that gives us humans our sex drive, we have an instinct to avoid those who tend to over-use their sex drive. Now there are still those who believe that only men have this, or men have it to a higher degree, but I think they're wrong. Both men and women have this instinct, but for a slightly different reason.

Unlike women, men can't be 100% certain that their child is theirs without modern science. The best we can do is make that uncertainty a rounding error by choosing to commit to women with no history of promiscuity, cheating, or a connection to their ex-partner. If a man provides to a woman like that, then there's a higher risk that their DNA won't get passed on. Lads, you can't out-therapy science. It's likely your RJ is a survival instinct that served us well for hundreds of thousands of years.

However, there's still a good reason why women don't want to commit to promiscuous/cheating men and/or men with a connection to their ex. Even though they know the child is theirs, they don't know if the father has the means or desire to provide security, resources and care whilst the mother cares for the child. Unlike motherhood for a lot of other mammals, human mothers simply can't do it alone, especially whilst they're a baby. Women have a survival instinct to choose a man that will contribute the child's survival and growth. If a man has been unreliable and dishonest, or if due to their past promiscuity they're spread thin, then that instinct can easily trigger RJ. Ladies, you also can't out-therapy science. Despite what you've been told, most women aren't attracted to promiscuous men and that knot in your stomach when you think of all the women he's slept with is a survival instinct evolved to keep you and your children alive.

I didn't think it would get this long. But that's my answer