As much as Americans revere the family, they differ sharply on how to define it.

New research being released Wednesday shows steadily increasing recognition of unmarried couples — gay and straight — as families. But there's a solid core resisting this trend who are more willing to include pets in their definition than same-sex partners.

How "family" is defined is a crucial question on many levels. Beyond the debate over same-sex marriage, it affects income tax filings, adoption and foster care practices, employee benefits, inheritance rights and countless other matters.

The new research on the topic is contained in a book-length study, "Counted Out: Same-Sex Relations and Americans' Definition of Family" and in a separate 2010 survey overseen by the book's lead author, Indiana University sociologist Brian Powell.

Between 2003 and 2010, three surveys conducted by Powell's team showed a significant shift toward counting same-sex couples with children as family — from 54 percent of respondents in 2003 to 68 percent in 2010. In all, more than 2,300 people were surveyed.

Powell linked the changing attitudes to a 10 percent rise between 2003 and 2010 in the share of survey respondents who reported having a gay friend or relative.

"This indicates a more open social environment in which individuals now feel more comfortable discussing and acknowledging sexuality," Powell said.

Only about one-third of those surveyed said they considered same-sex couples without children to be a family. And in 2006, when asked if gay couples and pets count as family, 30 percent said pets count but not gay couples.

"The sheer idea that gay couples are given less status than pets should give us pause," Powell said in an interview.

In the 2010 survey, 83 percent of the respondents said they perceived unmarried heterosexual couples with children as a family; only 40 percent extended that recognition to unmarried straight couples without children.

In line with several recent national opinion polls, Powell's 2010 survey showed a near-even split on same-sex marriage — with 52 percent supporting it and 48 percent opposed.

Even though five states and the District of Columbia now allow same-sex marriages, the federal government doesn't recognize them. The Census Bureau definition of "family" remains traditional: "A family is a group of two people or more (one of whom is the householder) related by birth, marriage, or adoption and residing together."

Many religious conservatives hope the government sticks by that definition, even in the face of shifts in public opinion.

"Same-sex marriage is a dangerous social experiment," said Glenn Stanton, director of family formation studies for Focus on the Family. "A lesbian couple who legally married in Massachusetts — are they family? We would say, 'Absolutely not.'"

Stanton said it was increasingly difficult to engage in serious debate on the definition question.

"We're moving in this headlong direction toward same-sex families without any intelligent discussion about whether it's actually good for the children and the adults," he said. "This whole issue has boiled down to, 'Are you a bigot or not?'"

The shifts described in Powell's research pleased Jennifer Chrisler, executive director of the Family Equality Council, which advocates on behalf of same-sex families.

"People are taking a more expansive view of what a family is," said Chrisler. "But for any family that doesn't fit the 1960s Ozzie and Harriet mold, slow and steady doesn't feel fast enough."

Chrisler and her wife, Cheryl Jacques, a former Massachusetts state senator, are raising twin boys.

The Family Equality Council has been lobbying on behalf of a bill pending in Congress that would prohibit states and child welfare agencies from denying adoption or foster care placements solely based on the sexual orientation or marital status of the potential parents.

The bill is targeted at states such as Florida, which bans gays and lesbians from adopting — a policy now being challenged in court.

The bill, introduced by Rep. Pete Stark, D-Calif., has been applauded by the Alternatives to Marriage Project because it encompasses single people as well as same-sex couples,

"I get frequent letters and e-mails from people who find the political rhetoric of 'family' to be extremely exclusive of singles," said the project's executive director, Nicky Grist. "For singles, it might be a code for 'You don't count.'"

For Powell, the major finding of his new research is the shifting view of same-sex families — which he compared to the gradual acceptance of interracial marriage.

"We envisage a day in the near future when same-sex families also will gain acceptance by a large plurality of the public," he wrote.

His book was published by the Russell Sage Foundation, a social science research center, as part of a series overseen by the American Sociological Association.

The surveys were conducted by telephone, among a random selection of households, and the characteristics of the samples were compared with census data to verify that they were representative. There were 712 interviews in 2003, 815 in 2006 and 830 this year.