SAN FRANCISCO (AP) – The growing acceptance of same-sex marriage in California reflects generational differences more than changes in individual attitudes, according to a study released Thursday.

A pair of political scientists who analyzed two decades worth of Field Polls on the subject found that age was the strongest factor influencing whether someone opposed gay unions, with people born in the 1970s and ’80s more than twice as likely to support them as those born before 1940.

“Californians born in each decade tend to be more accepting of gay relationships and more willing to grant them legal recognition than those born the decade before,” said the study’s authors, Gregory Lewis of George State University and Charles Gossett of California State Polytechnic University, Pomona.

“The findings suggest that same-sex couples will one day be allowed to wed in California, if not the rest of the country, as older generations die off,” said Mark DiCamillo, director of the Field Poll.

“It’s just a matter of time before a majority of California will be supportive of same-sex marriage,” DiCamillo said. “It may take 10 years to replace another decade within the age cohorts, but it’s clear every younger generation seems to be more accepting of that.”

Gossett and Lewis said that while differing opinions among generations accounted for two-thirds of the state’s increased support for same-sex marriage since 1985, the strong link between age and attitudes did not mean that Californians became more homophobic as they aged.

Even the oldest group surveyed demonstrated a small shift in opinion over the last 20 years. One-fourth of those born before 1940 supported same-sex marriage in 2006 compared to one-fifth in 1985, they found.

Researchers also asked respondents in the most recent survey to self-evaluate how their attitudes toward gays had changed since they were 18 years old. Forty percent of those age 55 and older said they had become more accepting, while eight percent reported they had become less accepting. More than half said their attitudes remained unchanged.

The professors said their “generational replacement” theory also may explain why California tends to be more tolerant of same-sex couples than other states: It has a comparatively young population. The state ranks sixth in the nation in the percentage of residents under 18.

In the 2006 survey, one-third of likely voters supported full marriage rights, another third endorsed the domestic partnership system the state has now and the last third opposed any relationship recognition for same-sex couples.

Besides age, politics and religion became increasingly strong predictors of whether survey respondents were likely to support same-sex marriage during the last 20 years, said Lewis and Gossett. In 1985, there was a 23 point gap between those who identified themselves as conservative or liberal, compared to a 61 point spread in 2006.

Because the study relied on long-term data instead of a one-time survey, it did not have a sampling margin of error, according to DiCamillo.