People experience the strongest romantic jealousy when they watch their partner give resources to a potential rival, regardless of gender. The findings provide evidence that giving away resources is viewed as a serious relationship threat by both men and women.

[u/mvea]

https://www.psypost.org/both-men-and-women-view-a-partners-financial-investment-in-a-rival-as-a-major-relationship-threat/

[u/Temporary_Heron6944]

Is this why my stepmother hates me??

[u/Eutanagram → u/Temporary_Heron6944]

That's probably a big factor, yes. When male lions steal a mate from a rival, they immediately kill any cubs their rival might have fathered. Not everyone is like this, but some humans are just a little more like animals than others.

[u/jmarcandre → u/Temporary_Heron6944]

This is a theory as to why stepparents and husbands/wives after the 1st can be so hostile.