A third of American adults now own a tablet computer, double the number of a year ago, the Pew Research Center said Monday.

The survey found that 56% of households earning more than $75,000 have a tablet, making it the biggest category of device owners. Adults in the 35-44 make up the biggest tablet-owning age group (49%).

By contrast, smartphones are much more popular with younger adults. Those in the 18- to 34-year-old category own the most smartphones in the U.S., Pew said last week.

Pew interviewed 2,252 adults, age 18 and older, in April and May. A Pew survey in May 2010 found that just 3% of adults owned a tablet. The tablet-owning number jumped to 8% in May 2011 and to 18% in April 2012. Tablet ownership reached 34% in the latest survey.

The latest survey found that 33% of respondents between age 18 to 24 owned a tablet and 37% of 25- to 34-year-olds owned one. Only 18% of respondents over age 65 own a tablet, the survey found.

Other findings in the study:

Tablet ownership among parents with minor children living at home rose from 26% in April 2012 to 50% in May.

Tablet ownership by adults in households making $75,000 a year rose from 34% to 56% over the past year.

Tablet ownership rose from 28% to 49% of college graduates in the same period.