The American Gaming Association found that 70 percent of all those who wager online first did so within the past two years, the Las Vegas Journal-Review reported.
The typical online gambler is male, under 40 and has an above-average income, the study found. Almost half say that they like gambling online because it's convenient, and four out of five live in states without legal casinos.
"It comes down to a convenience element," said research firm president Peter Hart. "The same reason people go to 7-Eleven, easy access and open 24-7, is the reason people gamble online. There is also the comfort, the relaxation and the anonymity. They don't have to deal with crowds."
While only 4 percent of the U.S. population gambles online, the number is growing fast, doubling in two years.