I have seen several cases of relationships in which the other person seems to be strongly mono by nature, and the other strongly poly by nature. They generally don't go very well, though they sometimes do: this seems to require the mono partner being of the type who can be okay with their partner dating others. Otherwise one of them is going to end up deeply unhappy, even if the relationship lasts.

Personal observation. Since the topic is deeply important to the mental health and happiness of a large fraction of the entire human population but sounds slightly silly, I would not particularly expect any significant experiments to have been done by academic science. Surveys of percentage actually practicing polyamory, yes, attempts to directly determine a wish / tendency / suitability in a general population, no. This is falsifiable if Carl or Jonah want to check cynicism, though I wouldn't be too surprised (the Kinsey Institute exists).