In my surveys of over 700 long-married older people, I often heard this lesson: “You don’t just marry a person; you marry his or her family.”

Indeed, many of the elders regarded this statement as a fundamental truth. Despite the fact that most dating couples do not spend much time thinking about their partner’s family, the elders tell you unequivocally: in-laws matter.

It’s no coincidence that popular culture focuses so heavily on in-law relationships, from the meddling mom and dad in “Everybody Loves Raymond” to the “Meet the Parents” movies.

These images reflect deep-seated worries about balancing loyalty to one’s spouse with life-long bonds of attachment and obligation to parents, siblings, and other kin.

This worry is not an irrational one; research also shows that in-law relations are a key determinant of marital happiness.

But what should you do? As I combed through hundreds of reports of in-law relations—ranging from loving and respectful relationships to “in-laws from hell”—I uncovered three terrific lessons for insulating your relationship from problems with one another’s’ families. These rules for in-law relations have been tested by hundreds of the oldest Americans for decades—given what’s at stake, we should pay close attention.

Rule # 1: Your loyalty is to your spouse

Life is full of difficult decisions in which no solution leaves everyone happy. Unfortunately, that’s exactly what a difficult in-law situation creates—a classic example of ambivalence that in a worst-case scenario may persist over years (or even a lifetime). But sometimes the elders cut through all the complexity and just tell you what to do. Here’s their advice on dealing with the supposed ambivalence of in-law relations:

In a conflict between your spouse and your family, support your spouse.

The elders are unequivocal; it is your duty to support your husband or wife and to manage your own family in a way that consistently conveys this fact. Further, you both must present a united front to both families, making it clear from the beginning that your spouse comes first.

In couples where this allegiance did not happen, marital problems swiftly followed. In fact, some of the bitterest disputes occurred over a spouse’s failure to support his or her partner. When I asked Erin, 66, to describe a conflict that came up in her marriage, she didn’t hesitate:

Oh yeah, his mother. A lot of conflict. I had the impression she didn’t like me very much. I could live with that, but my husband never stuck up for me, so we fought about it. The apron strings were tied to him, and you just didn’t go against Mommy. And we fought about it because he would say, “Oh you’re crazy, she never said that.” And I’d go, “I don’t believe you don’t believe me.” And arguments would start. And after it was over I’d say, you know, how stupid we’re arguing about this, God forbid we get divorced over her. My husband would never say anything like “Hey mom, that’s my wife, cool it.” I never got that.

So when there is conflict between your family and your spouse, don’t feel caught in the middle, because your place is on your spouse’s side. To do otherwise is to undermine the trust that is the underpinning of your marriage.

Rule # 2: Remind yourself why you are doing it

This tip from the elders is one that many have used like a mantra in difficult in-law situations. Tell yourself this: the effort to accommodate your partner’s family is one of the greatest gifts you can offer in marriage. You are used to putting up with your own relatives and you have accommodated to their quirks and foibles. But now you have to do it all over again. The closest thing to a “magic bullet” for motivating yourself to put the effort into in-law relations, the elders tell us, is to remember that you are doing it because you love your spouse.

Most important, by staying on good terms with his or her relatives, you are honoring and promoting your relationship in one of the best ways possible. Gwen, 94 and married 67 years, puts it clearly:

You may not like your mother-in-law or your father-in-law or your in-laws very much but you certainly can love them and stay close to them. Remember that they’re your loved one’s family. I learned to love them. I mean, I loved them because they were my husband’s parents and I loved him.

Rule # 3: Eliminate politics from discussion

Here’s a specific tip that could not be more relevant during this election season: Keep political arguments out of in-law relations. It can be the biggest bomb in the minefield, and the elders say that these conflicts are unnecessary. There is simply no need to attempt to engage your in-laws in political debates or to convert them.

Often, the urge is to make parents-in-law “really understand” what’s going on in society and to show them how irrational or wrong-headed they are politically. I heard many accounts of holiday dinners and family gatherings disrupted by debates over the President, the Congress, abortion, the death penalty, and on and on.

According to the elders, you may not be able to avoid conflict over your in-laws’ disapproval of your marriage, your job, your lifestyle, or how you raise your children. But you can make it a rule to take noisy and unnecessary political debates off the table. (Remember, we’re not talking here about a lively, enjoyable political discussion; I mean the kind that ends with slamming doors and a spouse crying in the car.)

Let’s return to Gwen for her advice. Gwen made in-law visits much more tolerable by following this lesson and cutting politics out of the interaction.

My husband didn’t care for my dad because my dad was a completely different kind of person compared to my husband. My dad was the boss of everybody and everything. He was never aggressive; he never hit us kids or my mother. But he was a total boss. What my dad said was law and order and we all knew it. And my husband was a gentle, soft-spoken, easy-going person who would rather die than make a fuss. He was a completely different personality. In particular, they didn’t see eye to eye about the government. My dad was a Democrat, my husband was a Republican. They’d get into those arguments.

So finally, I made the rule that there would be no discussions of politics when we were all together. And I said to my husband: “If Dad starts in about the Republicans, I’m going to walk out of the room and you come see what’s wrong with me because I don’t want to hear this anymore.” I guess that was the only problem in our early marriage. Of all the big decisions we had to make in marriage, I think the most important was deciding that I wasn’t going to listen to that problem between my father and my husband.

You may wish to apply this same rule to other “hot-button” issues (based on my own extended family, I’m tempted to include Red Sox versus Yankees...). When buttons are pushed on a repetitive and sensitive topic, leaving the room is an excellent—and potentially relationship-saving—option.