It's complicated.

The tax on Social Security benefits is triggered when a senior's wage, pension, investment and dividend income exceeds a set threshold. At that point, a senior who fills out a return by hand must complete an 18-line worksheet that ultimately determines how much of his or her otherwise tax-free Social Security benefits will become taxable. (Most tax software programs do the calculation automatically, so those who file that way may not even notice the extra tax.) 

For a single senior, each dollar of income over $25,000 makes 50 cents of his or her Social Security benefits taxable. For married couples, that threshold is $32,000.

Here's the article.