What's the secret to a happy marriage or long-term relationship? Many married people believe they know. Psychologists and marriage counselors are trained to dispense expert advice. But then there's brain science. Helen Fisher, a biological anthropologist and chief scientific advisor to Match.com actually took people experiencing various forms of romantic love and gave them brain scans to see what was going on in their heads. The result is her book Why We Love: The Nature and Chemistry of Romantic Love

We all know that first rush of falling in love, the thing that makes your love object completely irresistible--because it activates the same sections of the brain that addiction does, Fisher explains in a recent interview with Vox. As most married couples know, that addiction effect fades over time. So what then? How do you sustain a good partnership over the years? Here's what the brain science suggests, according to Fisher. As someone nearing my own 18th anniversary, I have to say her recommendations make a lot of sense.

1. See things from your partner's point of view, and let him or her know it.

One brain area that lights up for people in long relationships is the area associated with empathy, Fisher says. Of course, empathy is an important element of every type of relationship, including those you have with your friends and your co-workers or employees. But empathy is especially necessary when you're sharing your life with someone and making decisions together--inevitably, you will disagree about some important stuff and will have to work it out between you. Understanding why your partner feels different than you do and wants something other than what you want is a necessary first step to navigating those differences. Letting your partner know that you understand is just as important.

2. Manage your own stress and emotions.

There's a great moment in the 1989 movie Immediate Family when a young unmarried woman, asks a happily married older one the secret to a long marriage. Part of the answer: "Only one person gets to be crazy at a time." 

That's supported by the science--Fisher says the part of the brain that controls stress and emotions is activated in people with long-term partners, and it makes perfect sense. In any lengthy relationship, there will be many times when you have to put your own frustrations and desires aside to prioritize your partner's, because you know that at other times your partner will do the same for you. And there are many, many times when you have to refrain from saying something you're thinking because you know it won't help your partner, your relationship, or you. Other times, you do need to express what you feel to keep the relationship alive. It takes judgment to know the difference.

3. Focus on the positive.

Fisher says that people in long relationships have heightened activity in a brain region associated with what she calls "positive illusion." That's her term for the ability to overlook what you don't like about something or someone and focus on what you do like.

Now, I can't stress enough that positive illusion can be a very dangerous thing in certain relationships. If your partner has a harmful addiction, or is physically or emotionally abusive, or is dishonest, or endangers you in any way, then you absolutely must focus on those negative things and either resolve the issue or end the relationship.

That said, Fisher is right that the way you see your relationship has a lot to do with what you choose to focus on and what you choose to ignore. Is it more important that your partner brought you flowers or forgot to do the dishes? That your partner knows how to make you laugh or that he or she doesn't dress the way you like? The more you positively reinforce the things your partner does that make you happy, the more often those things will happen, and the happier you'll be. And that happiness, that sense that your life is a whole lot better with your partner than without is what will get you through the tough times, and help you stay together for years to come.