The percentage of the internet users in the U.S and the U.K. who play social games for periods greater than 15 minutes a week have nearly a doubled, according to a study by PopCap Games.

About 41 percent of players now play for more than 15 minutes, compared 24 percent last year. PopCap Games, a division of Electronic Arts, is one of the most well-recognized names in the field of casual video games. Founded in 2000, the company has a worldwide staff consisting of more than 500 people. Its flagship franchise Bejeweled has sold more than 50 million units worldwide.

Nearly 120 million people are now playing social games, a 71 percent increase in growth over time. More intriguingly, the survey also reports the likelihood of social gamers purchasing in-game content with real money has increased to 46 percent from 32 percent in January 2010. About 31 million players were reported to have purchased in-game currency of some variety.

While the survey reports that female social gamers continue to outnumber their masculine counterparts (54 percent vs. 46 percent in the U.S and 58 percent vs 42 percent in the U.K.), it appears that male consumers are more likely to make in-game purchases. Additionally, social gaming has experienced an influx of younger players; the average age of a social gamer has declined from 43 to 39 years.

This research, which was conducted by Information Solutions Group exclusively for PopCap Games, is based on the results of 1,201 online surveys completed by members of Toluna’s internet ePanel between Sept. 15 and Sept. 22. The surveyors said 801 of the respondents originated from the U.S. while 400 were from the U.K.

The full report is available for download at 2011 PopCap Social Gaming Research.