When falling in love or fancying someone, one emotion can dominate over the rest: jealousy. Some may try to play it cool and act aloof, but seeing - or even thinking - of a romantic partner engaging with others can lead people to act completely out of character. The green-eyed monster can hijack thoughts for days to weeks on end, spending precious energy ruminating on situations that may never arise. So why is it that humans feel jealousy? Do people experience this emotion differently? And are there ways to stop it? CrowdScience presenter Caroline Steel sets about answering these questions from listener Odile in France, who has struggled with all-consuming jealousy in some romantic relationships. She hears about a kind of monkey who get jealous of their own reflection from Professor Karen Bales at University California Davis. A trip to ZSL London Zoo sees more monkeys, but these are more bothered about protecting the vital friendships which aid their survival. Dr Alex Mielke from the University of St Andrews explains how these interactions can give us an insight into why jealousy exists. Some of us get more jealous than others and are more likely to act out of character when the green-eyed monster takes hold. Caroline completes a detailed questionnaire to see how jealous she really is, and gets advice from Julia in South Africa, who is in a polygamous marriage and has had to handle romantic jealousy. The nature-nurture balance of jealousy is untangled by geneticist Dr Laura Wesseldijk from Amsterdam UMC (who reveals some surprising information about the first author on her research paper…) and psychologist Dr Johan Ahlen from the Karolinska Institute rounds off the programme discussing how the future of jealousy management could look like for those who struggle. Presenter: Caroline Steel Producer: Julia Ravey