WASHINGTON (Reuters) - A record 20 percent of adult Americans, or 42 million people, have never married, marking a U.S. demographic and social shift, according to an analysis released on Wednesday.

The rise in never-married adults is caused by several factors, including later ages for marriage and more people living together and raising children outside of wedlock, the report by the Pew Research Center said.

In 2012, 23 percent of men and 17 percent of women 25 and older had never been married, marking a widening gap between the sexes. In 1960, 10 percent of men and 8 percent of women had never married, said the Pew report.

"Shifting public attitudes, hard economic times and changing demographic patterns may all be contributing to the rising share of never-married adults," Pew said. The analysis was based on Census Bureau data and a Pew survey.

The trend is especially pronounced among black Americans. Thirty-six percent of blacks had not been married in 2012, four times the level in 1960.

The share of never-married adults for whites has roughly doubled over the same period to 16 percent and for Hispanics to 26 percent, the Pew survey said.

About half, or 53 percent, of never-married adults said they would like to marry eventually, down from 61 percent in 2010, Pew said.

Men and women are looking for different qualities in potential spouses. Among never-married women, 78 percent say finding someone with a steady job would be very important.

For 70 percent of men, sharing similar views about raising children is more important than finding someone with a steady job.

Pew said the percentage of never-married adults has climbed as the gap in earnings between men and women has narrowed since 1980.

The median hourly wages for men 25 to 34 years old are down by a fifth over the same period. Young men's participation in the work force has also dropped since 1960.

The median age at first marriage is 27 for women and 29 for men, up from 20 and 23 respectively in 1960.

The Pew survey was carried out from May 22 to 25 and from May 29 to June 1 among 2,003 adults 18 and older. The margin of error is 2.5 percentage points.

(Reporting by Ian Simpson; Editing by Doina Chiacu)