Demonstrators on both sides of the gay marriage debate protest outside a Detroit federal courthouse on Tuesday, Feb. 25, 2014, during the first day of trial testimony in a case challenging Michigan's voter-approved same-sex marriage ban.

DETROIT, MI -- Gay couples benefit economically when they are allowed to marry, said a demographer who testified on the third day of the trial in a Hazel Park couple's challenge of Michigan's gay marriage ban.

Gary Gates, a demographer of the Los Angeles-based Williams Institute who studies gay, lesbian, bisexual and transgender populations, offered a series of statistics showing growing numbers of American individuals, couples and parents who identify as LGBT.

- In Michigan, around 287,000 adults and 14,600 couples identified as LGBT in 2012, Gates said.

- He said studies show 1.1 percent of the U.S. population self-identified as gay, lesbian or bisexual in 1992. That number went up to 3.4 percent in 2012.

- Gates said those numbers are expected to continue to grow as younger generations, who self-identify as LGBT in higher proportions, get older. "There will likely be more people who identify as same-sex couples and more who identify as LGBT in Michigan in the future," he said.

- "There are about 2,650 same sex couples raising approximately 5,300 children" in Michigan, according to 2010 U.S. Census numbers," Gates said.

- About 14 percent of U.S. same-sex couples who are parents adopted their children, compared to 3 percent of heterosexual couples, Gates said. In Michigan,11 percent of same-sex couples with children adopted, compared to 4 percent of heterosexual couples, he said.

(More stats gathered by Gates are available here.)

Gates on Thursday also spoke on the economic benefits of marriage.

Attorneys for the state objected to that portion of his testimony because Gates is not an economist, but Judge Bernard Friedman allowed him to continue.

Gates said married couples are more likely to have health insurance, and that “there's general evidence that married couples tend to do better with income when you compare them to unmarried individuals."

Gates' statistics are largely based on U.S. Census numbers, and state attorneys sought to weaken his testimony, as they have with earlier witnesses, by pointing out deficiencies in Census data and calling attention to relatively small sample sizes in other studies related to LGBT issues.

State attorney Michelle Brya directly also directly asked Gates, in an apparent attempt to show potential bias, if he personally thinks gay couples in Michigan should be allowed to marry.

"I do," Gates said.

Gates was the fourth expert witness to testify in the trial, which began Tuesday.

Jayne Rowse and April DeBoer, the two nurses from Hazel park who sued the state, are seeking the right to marry and adopt children together, arguing that the Michigan's gay marriage ban, approved by voters in 2004, "enshrined discrimination in the state constitution."

Lawyers for the state have claimed its defense of the gay marriage ban is not an attack on the gay and lesbian community, and that voters made a rational decision to "encourage traditional marriage."

Rowse and DeBoer, who have lived together for six years, initially raised their lawsuit because they can't jointly adopt their three children.

Rowse has adopted two children, Jacob and Nolan, and DeBoer has adopted one, Ryanne. They consider themselves married. But without a legal marriage in Michigan, if one parent were to die, the other may not get custody of all three children.

When they appeared in federal court challenging state adoption laws last year, Friedman indicated that they might have a better chance if they challenged the state's gay-marriage ban.

So they expanded the lawsuit to do so, bringing them under the national spotlight. Friedman in October ordered the case to trial.

On the first day of trial, the court heard from psychologist David Brodzinsky and sociologist Michael Rosenfeld, who each conducted studies indicating that children raised by same-sex couples do not suffer disadvantages when compared to children of heterosexual couples in similar circumstances.

They both criticized a study by University of Texas sociologist Mark Regnerus, who conducted his own study indicating there are disadvantages for children of gay couples, claiming he failed to control for economic and social variables that influence child outcomes.

Regnerus is expected to be called to the stand by the state later in the trial.

On the second day of trial, University of Michigan law professor with expertise the foster system Vivek Sankara testified that Michigan law doesn't guarantee a way for gay couples to designate custody of their children in case of the death of one parent.

And Gates offered his statistical analysis on the third day of the trial.

All the witnesses so far have been called by the plaintiffs.

Another is set to be called to the stand on Friday: Nancy Cott, a Harvard University history professor who specializes in gender and sexuality.

Next week, Oakland County Clerk Lisa Brown, who sides with the plaintiffs and wants to issue marriage licenses to gay couples, will introduce her witnesses.

The state will then present its witnesses.

Thursday's testimony lasted less than two hours.

George Chauncey, a Yale University historian, was supposed to also supposed to testify Thursday, but Friedman said he was "unexpectedly unavailable."

He instead has submitted a report to the court detailing his opinions on the history of discrimination against homosexuals in the U.S.

It appears the state won't get a chance to cross-examine him. The judge may decide not to allow the report to affect his decision.

But he did allow the report to be entered into public record.

"History has vindicated the judges who had the courage and foresight to uphold the Constituitional rights of disfavored minorities in the face of majoritarian hostility," Chauncey wrote.

"Among the many products of the legacy of discrimination in the twentieth century, the most conspicuous today include Congress' repeated failure to enact federal legislative protections for gay and lesbian people in housing, employment and public accommodations; the numerous state statures and constitutional amendments that brand gay men and lesbians a second-class citizens by denying them the right to marry the person they love... Like other minority groups, gay men and lesbians often must rely on judicial decisions to secure equal rights."

Follow MLive Detroit reporter Khalil AlHajal on Twitter @DetroitKhalil or on Facebook at Detroit Khalil. He can be reached at kalhajal@mlive.com or 313-643-0527.