SAN FRANCISCO (AP) – The U.S. Census Bureau is making an unprecedented effort to include same-sex couples in next year’s national population count, but legally married same-sex couples won’t show up as such in the official once-a-decade tally, bureau representatives said Thursday.

Statistical problems related to the development of the 2010 census form and the evolving legal state of same-sex relationships led Census officials to conclude that trying to include married gay couples in the overall snapshot of household marital status could yield an inaccurate number, said Gary Gates, a University of California, Los Angeles demographer who has been advising the bureau on gay issues.

Instead, same-sex married couples will be added into the category for unmarried partners, just as they were for the 2000 census. But in a marked policy departure, the agency plans to make the data on same-sex couples who described themselves as married available on a state-by-state basis.

“The Bureau has decided to give us the information, but be a little cautious,” Gates said.

The decision to develop separate sets of numbers was a compromise position that was “less about politics and more about accurate data,” he said.

Gates stressed that it was important for gay couples to participate in the census, noting that information drawn from the last one had been used in lawsuits dealing with same-sex marriage and to lobby congressional representatives who may wrongly assume they do not have many gay constituents.

Because same-sex marriages were not legal in any U.S. state a decade ago, the 2010 census is the first for which the bureau has wrestled with how to count married same-sex couples. Now six U.S. states allow same-sex marriages.

In June, census officials announced that they would make the attempt, reversing an earlier decision made under the Bush administration.

Since then, however, it’s become clearer that a wildly inflated number could be produced if the number of heads of household who said they lived with another adult of the same sex, and described that person as a husband or wife, were only counted.

Some couples in civil unions or domestic partnerships, or who live as spouses in states where gay couples have no spousal rights, have tended in past surveys to identify themselves as husbands or wives anyway, according to Gates.

The annual American Community Survey the bureau produced for 2008, for example, had 150,000 married same-sex couples spread across every U.S. state, even though only two states – Massachusetts and for a 5-month period, California – allowed same-sex marriages. Gates estimates there are probably no more than 35,000 legally married gay couples in the country now.

Undercounting same-sex couples also remains a significant concern, Gates said, since some couples may not be living openly and fear discrimination.