Level Gap Experience Penalty

It’s well-known that if a player you are partied with uses a pokemon higher level than you by a certain amount, you’ll receive less experience for KOs they score when you’re not close by. However, despite the major impact this has on party play, the actual mechanics and level threshold behind how this works aren’t well documented. I’ve heard a different answer from every person I’ve asked since all the way back in 2014.

So could I get some staff clarification on how party EXP distribution–or more specifically EXP distribution in parties with imbalanced levels–works? It would be much appreciated.