I think there's a big factor that makes these studies weak in nature. Women are less likely to remarry for a second or third time and are much more likely to cohabitate long term, so this breaks the numbers quite a bit. A major factor of that is keeping divorce settlements in place that support earlier children. It could be that if she has two rounds of children and is a primary caregiver, it might be significantly more work and stress.

Also because of all the social pressures, relationships with the woman older are more likely to happen with both partners who are neurodivergent, meaning socially resistant, making their supportive communities smaller. All neurodivergent people die younger and we're at the tip of the iceberg figuring the whys and how's of that one. This thing is so multidimensional, we just don’t know enough yet.