Second Earner Penalty

Every model leaves things out. Here is what this one does not see:

• Pension contributions, employer benefits, and social-security/state-pension credits earned by a second income are entirely absent. Stopping work can carry a large invisible long-run cost that a one-year cash snapshot can't see.
• Tax is applied as a single flat marginal rate on the whole secondary gross. In many systems a second earner's own income is spread across multiple brackets, or taxed under joint-filing rules, rather than landing on one clean marginal slice. The true hourly wage shown here could be higher or lower than reality depending on which way the real tax code bends.
• Induced costs (childcare, commute, convenience) are assumed to scale linearly and vanish completely without paid work. There's no partial substitute (a relative helping with childcare, say) and no acknowledgment that some of these costs serve non-work purposes too.
• This is a one-year cash snapshot. It does not compound the net cash added if invested, and it says nothing about how forgone income today changes future earning power: the raises and promotions that stepping back from paid work is often said to cost.