Gig economy workers could get short changed when it comes to their Social Security checks in retirement.

The growing ranks of people who earn money on apps like Uber, Airbnb and Task Rabbit are more likely than traditional employees to misreport their income. That could mean a smaller Social Security check down the road because the benefits are calculated from reported lifetime earnings.

That's the finding from a recent study by Caroline Bruckner, managing director of the Kogod Tax Policy Center at American University and economist Thomas L. Hungerford. The research was funded by the Center for Retirement Research at Boston College.

In 2014, independent contractors didn't pay $3.9 billion in Social Security contributions that they should have, and on-demand workers didn't pay $2 billion, according to estimations by the study's authors.

These estimated shortfalls are just for one year and are likely to grow, Bruckner said.

In 2017, Brad Smith, the CEO of Intuit, which owns TurboTax, estimated that a third of the U.S. workforce earned money from the gig economy — and that share will increase to 43% by 2020.

At the same time, IRS records show the number of filers penalized for underpaying estimated taxes jumped to 10 million in 2015 from 7.2 million in 2010, according to a recent report by The Wall Street Journal.