The top 50 grossing iOS mobile games in the U.S. accounted for 76% of the total revenue in 2017, but they usually don’t stay in the top 50 for long. That’s according to a new study by data company Apptopia.

Apptopia looked at the highest grossing games for the iOS App Store in the U.S. from 2014 to 2017. It studied about 800,000 titles over the four year period and found the average lifespan for games within the top 50 was 27.75 days. About 33% of games that break into the top 50 only stay there for one day.

Only six titles stayed within the top 50 for the entire four year period — “Game of War: Fire Age,” “DoubleDown Casino Slots & More,” “Big Fish Casino: Slots & Games,” “Slotomania Slots: Vegas Casino,” “Candy Crush Saga,” and “Clash of Clans.”

“Since we’re talking money, it’s no surprise that three of these are casino games,” Apptopia said.

iPhone users are reportedly spending more money on apps than ever. They spent an estimated average of $58 on in-app purchases and premium apps in 2017, according to intelligence firm Sensor Tower. That’s a 23% increase over the year before. More than half of that money (62%) went to mobile games.

Many of the top grossing games have one thing in common — balance between player gender. Apptopia chief operating officer Jonathan Kay said in a recent blog post dominating the male gaming market is no longer enough to make it to the top.

“The top grossing mobile games got there because they understand how to incorporate game mechanics and themes that people of all ages and sexes can enjoy,” he wrote. “While identifying a niche can be lucrative, the serious money is made by catering across demographics.”