Over the last ten years, increasing evidence has mounted for why mammals choose, keep, and cheat on their mates.
MHC, or Major Histocompatibility, is almost half the story. Histocompatibility means “cell compatibility”. Organ transplant patients need tissues that are as similar to their own cells as possible or the body will detect the transplanted organs as foreign and kill them off. However, we need robust and varied MHC markers to protect us from foreign invaders, or to alert immune surveillance cells that one of our own cells has gone rogue, become cancerous, and needs to be destroyed. See “Smelling Cancer” video below. Our MHC proteins are as unique as fingerprints and are encoded in our genes. Horses have very different MHC proteins than humans. Our brothers and sisters have MHC genes that are very similar to our own.
When we kiss or hug another person, we taste and smell them. It turns out that there is a range of MHC distance that is most appealing for long term romance. Kiss a horse, and you will probably not want to stay married to them. Ditto for kissing your sibling. But kiss someone with a human MHC that is very different than your own, and you will be more likely to produce children with robust and varied MHC genes, and you will be more likely to stay married and faithful to them over the long haul.
Vive le difference. And see the video below.