Increasing your 401k deduction will your take-home pay and your federal taxes in the current year.

Decrease your take-home pay and decrease your federal income taxes in the current year. A 401k allows you to deduct earnings from your current income, and put that money in an account that cannot be opened until around retirement. The money put into a 401k, and the returns earned on that money through interest or investments, is not taxed as income until you take it out after retirement. This means that, when you put some of your earnings in a 401k, your take-home pay is lower for that year (you can't spend that money) and your income tax is reduced.