Opinions expressed on Twitter differ from public opinion measured by surveys on key political events and policy issues, reflecting that users of the Internet service are demographically very different from the public, according to a study by Pew Research Center.

Reactions on Twitter also tend to be more negative than in national polls. "Often it is the overall negativity that stands out," Pew Research said Monday. In the run-up to the U.S. presidential elections last year, negative comments exceeded positive comments for both candidates by a wide margin throughout the fall campaign season.

Responses on Twitter are also not always more liberal than those of the public.

After President Obama's re-election on Nov. 6, for example, 77% of post-election Twitter comments about the outcome were positive about Obama's victory while 23% were negative. A survey of voters by Pew in the days following the U.S. presidential election found a more mixed reaction -- 52% said they were happy about Obama's re-election while 45% were unhappy.

A pro-Democratic or liberal tilt in tweets was however not evident in reactions to Obama's second inaugural address in January and his State of the Union address last year, Pew Research said.

The year-long study by the center compared the results of U.S. national polls to the "tone of tweets" in response to eight major news events.

The inconsistency in the views expressed on Twitter and public opinion reflects that "those who get news on Twitter -- and particularly those who tweet news -- are very different demographically from the public," according to Pew.

Twitter users are still a small percentage of the public. In a survey on news consumption by Pew released in September, only 13% of adults said they ever use Twitter or read Twitter messages, and only 3% said they regularly or sometimes tweet or retweet news or news headlines on Twitter.

Users of Twitter are also not representative of the public, as they are considerably younger than the general public and more likely to be Democrats or lean toward the Democratic Party, Pew said.

A study of tweets can, however, bring in perspectives from people under the age of 18 who are not included in national surveys that are limited to adults aged 18 and older. Twitter conversations also may include those living outside the U.S., Pew said.

The data on Twitter responses came from an analysis of publicly available tweets, which were identified using multiple search terms for each subject. The parallel opinion surveys were conducted during the same general time period as the Twitter data were aggregated, Pew said.

John Ribeiro covers outsourcing and general technology breaking news from India for The IDG News Service. Follow John on Twitter at @Johnribeiro. John's e-mail address is john_ribeiro@idg.com