In 2017, Uber began letting riders tip riders in its app. For the vast majority of trips, riders decline the option. According to a research analysis (paywall) of 40 million trips in 2017, only 16% of trips are tipped and 60% of riders never tip.

Yet there are certain features of a trip that make a tip more likely. A higher rated driver, a longer trip, a richer rider, and a repeat customer all sizably increase the likelihood and size of the gratuity.

It also helps if the driver is a young woman, and the rider is a man. Even after accounting for factors like the length and time of the trip, the researchers found that the expected tip is greater for female drivers than male drivers, and that this is particularly true for young women. While women drivers over 65 get 2% more, on average, than male drivers over 65, women between ages 21and 25 receive over 6% more than men of the same age. Female riders also give women more money, but the difference isn’t as drastic (women are generally less likely to tip).