Twenty years ago this month, Marcia Kadish and Tanya McCloskey exchanged wedding vows at Cambridge City Hall in Massachusetts and became the first same-sex couple to legally marry in the United States.

The couple had been together since 1986, but their decision to wed was radical for its time. In 2004, only 31% of Americans supported same-sex marriage, while 60% were opposed, according to a Pew Research Center poll.

Much of that opposition was fueled by fears that expanding the definition of marriage beyond the traditional union of a man and a women would undermine the institution and be destabilizing to families. Researchers at the Rand Corp. decided to find out if those predictions turned out to be true.

A team from the Santa Monica-based think tank spent a year poring over the data. The result is a 186-page report that should be reassuring to supporters of marriage equality.

“If there were negative consequences in the last 20 years of the decision to legalize marriage for same-sex couples, no one has yet been able to measure them,” said Benjamin Karney, an adjunct behavioral scientist at Rand.

Karney, who is also a social psychologist at UCLA, led the report with Melanie Zaber, a labor economist and economic demographer at Rand. They spoke with The Times about what they learned.

Does marriage make people better off?

Benjamin Karney: On average, yes. People who are married experience fewer health problems, they live years longer, they make more money, and they accumulate more wealth than people who marry and divorce or who don’t marry at all. People who are married also experience more stable and positive psychological health, and they have sex more frequently than people who are not married.

All those benefits accrue primarily to people who are in happy marriages. Unhappy marriage is very, very harmful. But most people who are married are happy — that’s why they stay married.

What prompted you to examine same-sex marriage now?

BK: At the time that these policies were changing, there were a lot of arguments on both sides about whether the consequences would be positive or negative. Twenty years is a long time, and during that time, a lot of research has been conducted. It seemed like a good time to ask the question: What did happen as a consequence of legalizing marriage for same-sex couples? So that’s one reason.

The second reason is that in the Dobbs decision that overturned Roe vs. Wade, Justice Clarence Thomas in his concurring opinion said explicitly that this Supreme Court should consider reviewing and potentially overturning other decisions, and he named the 2015 Obergefell vs. Hodges decision that legalized marriage for same-sex couples by name. Given that people may be wondering about the merits of that decision, it seemed like a good time to evaluate the consequences of that decision, and that’s what we’ve done.

What did you find?

BK: We found 96 studies across a range of disciplines. Some are in economics. Some are in psychology. Some are in medicine. Some are in public health.

Melanie Zaber: We wanted it to be research that actually measured something. There were a number of more qualitative or theoretical or legal arguments that we excluded.

BK: What I found most notable is that all of the studies drew the same conclusions: There was either no effect or beneficial effects on any outcome you could look at. That’s 20 years of research, 96 studies, and no harms.

Does it seem plausible that the results could be so one-sided?

BK: I was not surprised. There’s a lot of good theory in family science and relationship science to argue that if you extend rights to a group that’s been stigmatized, that group should do better, and the majority group should not be affected. Indeed, that’s what we found.

MZ: I don’t find it particularly surprising. When we say there are no harms, that doesn’t mean everything’s coming up sunshine and roses — it means sunshine and roses or nothing. In this case, where the prediction was something negative, then nothing still feels like sunshine and roses.

What sorts of things did these studies measure?

BK: There were three general categories. The largest group was looking at outcomes for LGBT individuals and same-sex couples. The second bucket looked at the children of same-sex parents. And the third bucket was the effect on everybody else.

There was no evidence of harms anywhere.

That’s interesting because opponents of these policy changes very strongly — and very explicitly — predicted there would be harms. They predicted it in front of the Supreme Court, arguing that if we allow same-sex couples to marry, the consequences for the country will be negative and severe and unavoidable and irreversible.

Who benefits the most from legalizing same-sex marriage?

BK: Same-sex couples. Their relationships last longer when they are able to marry and cement their commitment. Their incomes go up. Their mental health improves.

That mental health improvement extends to LGBT individuals whether or not they are married. Even if you’re not married, if you’re a member of a sexual minority and live in a world that validates same-sex relationships, that relieves a stressor and has measurable benefits on physical and mental health.

What’s behind these improvements?

BK: The effects on health seem like they operate partly through employer-based health insurance being extended to spouses.

The mechanisms for mental health have been described by minority stress theory. Living in a society that is constantly sending you a message that you are less worthy of equal treatment is stressful, partly because it leads to discrimination. Being the target of discrimination is stressful, and that stress has real mental and physical consequences.

You found 96 studies about gay marriage. Why did you conduct your own research as well?

MZ: Some of those studies were conducted when only a few states had marriage for same-sex couples. A state like West Virginia or Wyoming might say, “Well that’s all well and good that you have evidence from Massachusetts or Vermont, but New England isn’t the center of the universe.”

By looking at a broader range of years, we’re better able to capture some of those states that did allow same-sex couples to marry but weren’t among the first to do so. We have reason to think those states may be very different environments. Our approach was to use each state as a quasi-experiment.

What did all that data tell you?

MZ: The headline from our new analysis is no negative impacts and some positive ones.

We see an increase in marriage, and that increase is driven not just by newly marrying same-sex couples, but also by an increase in marriage among different-sex couples. That was a bit surprising to us.

What do you think was going on?

MZ: There are a few different mechanisms for this, none of which we can explicitly test.

One could be allyship. There are individuals who identify as cisgender straight individuals, but they want to show their allyship so they delay marriage until everyone’s able to marry.

There’s an increasing number of individuals who identify as bisexual in the United States. Even if they’re marrying a different-sex partner, they may be trying to have validation of their broader identity.

The argument we find most compelling is that having people loudly clamoring for all the great things that come along with marriage made people in the broader population say, “Oh hey, getting married means people can go visit me in the hospital, and that if I’m in an accident there’s no concern about who my property will go to, and we have more access to health insurance.” Talking about that may have made some people realize, “You know, marriage actually is pretty helpful.”

BK: If you hear about a restaurant that everyone’s trying to get into, you want to eat at that restaurant.

MZ: That is an excellent way of putting it!

Do you think this research will persuade those who were concerned that same-sex marriage would have terrible consequences?

MZ: That’s our goal — to put evidence out to the public so policymakers can make informed choices.

BK: I’d like to believe so. At the time those arguments were made, they were speculative. People were trying to predict the future. Now we don’t have to predict the future. Twenty years have passed and we have the data. We can document what has happened.

This interview has been edited for length and clarity.