If Mark Zuckerberg and a janitor who works at Facebook’s headquarters each received a speeding ticket while driving home from work, they’d each owe the government the same amount of money. Mr. Zuckerberg wouldn’t bat an eye.

The janitor is another story.

For people living on the economic margins, even minor offenses can impose crushing financial obligations, trapping them in a cycle of debt and incarceration for nonpayment. In Ferguson, Mo., for example, a single $151 parking violation sent a black woman struggling with homelessness into a seven-year odyssey of court appearances, arrest warrants and jail time connected to her inability to pay.

Across America, one-size-fits-all fines are the norm, which I demonstrate in an article for the University of Chicago Law Review. Where judges do have wiggle room to choose the size of a fine, mandatory minimums and maximums often tie their hands. Some states even prohibit consideration of a person’s income. And when courts are allowed to take finances into account, they frequently fail to do so.

Other places have saner methods. Finland and Argentina, for example, have tailored fines to income for almost 100 years. The most common model, the “day fine,” scales sanctions to a person’s daily wage. A small offense like littering might cost a fraction of a day’s pay. A serious crime might swallow a month’s paycheck. Everyone pays the same proportion of their income.