I don't follow. If you encourage both to work, don't you get more total productivity? How does productivity increase by incentivizing one to leave the workforce?
And by encouraging both to work, you'll get more total tax revenue.
> How does productivity increase by incentivizing one to leave the workforce?
Maybe it's a long term strategy? The tax you'll levy on the kid (as the kid and later as adult) is expected to be much more than what you can levy on one partner (during child rearing)?
Because married couples form a household and it allows them to share child care and work as they see fit.
If you tax individuals, you're encouraging both to earn the same amount of money.
If you tax couples, it encourages the higher earner to keep working, thus you have a higher overall productivity.
Thus you have freedom and higher overall productivity in favor of shared tax burden.