Hi Lea,
I will answer your question based on NY law - since that is where we are based.
In NY, pension assets earned during your marriage are considered marital, or jointly-owned, property. The presumption is that, during your marriage, you functioned as a team. You may have decided that one person should contribute the maximum to their pension plan, because their company has a good match - and the other would contribute a smaller amount.
At the end of the marriage, the court (or the two of you, in a mediated agreement) would generally do as follows:
1 - figure out what you each had, as far as retirement assets, on or as close to the date of your marriage, as is possible.
2 - pick an end date - could be the date you separated, the date one of you files an action for divorce, the date you sign your mediation agreement.
3 - get values of pension assets on the end date.
4 - subtract #1 from #3. This is the value earned during your marriage.
5 - Then add each of your values together, and divide by 2. This is what each of you should walk away with, at the end of your divorce settlement.
Finally - pension assets usually need a court order, for them to be divided.
Hope that helps,
Rachel
PS - If you are in a different state - then you should check with someone there.