Two 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 Georgia 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 gay 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.
The researchers 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.
The California analysis mirrors a national Pew Research Center study released in January that found the younger people were, the more likely they were to support marriage rights for gay men and lesbians.
In that poll, 47 percent of those age 18 to 25 favored allowing gays and lesbians to marry, compared to 30 percent of those 26 and older.
Besides age, politics and religion became increasingly strong predictors of whether survey respondents were likely to support gay marriage during the last 20 years, Lewis and Gossett said.
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.