The posts above have described, generally accurately, the way that the tax system works. However, none really described the difference in the likelihood of refunds between a single person and a married person.
The amount of taxes you pay is determined by the tax return you file at the end of the year. The amount of taxes withheld from your earnings during the year may be more, or less, than the amount of taxes shown on your return; if more was withheld, you get a refund, and if less was withheld, you owe money. The DEFAULT amount of withholding (for your actual number of exemptions and marital status) is designed to be roughly equal to your total taxes shown on your return, but your actual taxes can vary due to deductions etc as others have noted. And as others have noted, you can have the amount of withholding increased or decreased by declaring more or fewer exemptions, as well as having an additional amount withheld from each paycheck.
A single person pays higher tax rates for the same income than a married couple. For example, if you have a 2009 taxable income (line 43 of the 1040 tax return) of $50,000, the amount of tax is $8694 if you are single, and $6669 if you are married. For this reason, the DEFAULT amount of withholding tax is less for a single person than a married person. Either one could pay tax at the end of the year, or get a refund at the end of the year.
The highest likelihood of owing taxes at the end of the year is not for a single person or for a one-income married person, but rather, for a two-income married couple, assuming the default for withholding tax is used. That's because of our system of graduated tax rates (higher income gets taxed at a higher rate) and the fact that withholding tax is calculated on each person as though that is the sole income of the family. A single person earning $100,000 gets withholding tax calculated as though that is his family income for the year. A married couple with one spouse working and earning $100,000 gets withholding tax calculated as though that is their family income for the year. Both are likely to be fairly accurate, before considering exemptions and deductions. However, a married couple with both spouses working and each earning $50,000 gets withholding tax calculated as though $50,000 is their family income for the year, and this is at a lower tax rate percentage than a family with $100,000 of income. Because the two incomes, together, result in a family income of $100,000, that pushes them into a higher tax bracket, and there is a greater likelihood of owing tax at the end of the year than for a single person or a married couple with one wage earner.
Hope that makes sense.
I don't get that rational behind it though... I thought the tax return was to return a small portion of the income which they take from each of your checks during the year.
Your terminology is not correct, which is probably why you are getting confused. The tax
return is the tax FORM that you return to the government, showing your income and deductions. The tax
refund is the amount that the government sends you back if you paid more taxes in withholding from your earnings than you actually owed at the end of the year. You may get a refund, or you may owe money.