Married couples in their later years often show a great deal of affection,says best-selling author Maggie Scarf, 77, who has spent more than 30 years studying relationships.

"There's intimacy.There is pleasure in each other's company. They say to each other, 'I love you more than ever.' " Scarf's new book is "September Songs: The Good News About Marriage in the Later Years," out this week. She has been married for 55 years to Herb Scarf, 78, a Yale professor. They have three daughters. Scarf shared her insights with USA Today.

Question: How would you describe your marriage?

Answer: Like any other couple, we've had our ups and downs at times. We haven't ridden through 55 years on a cloud of bliss. But the fact is we have always remained committed to each other. We have a lot of fun. My husband has a tremendous sense of humor, and we laugh a lot. He is my best buddy.

Herb is the person who knows everything about me. And I know everything about him. At least we think we do.

Q: What is the U-shaped curve in marriage that you describe in the book?

A: There have been pretty influential studies over the past 40 years that show a couple's contentment is at its highest in the earliest phase of marriage.

Then you get to know the other person's foibles and faults. Kids come along and you lose sleep and you want the other person to do more than they are doing. Then you are negotiating on a daily basis with your adolescents, and your sense of contentment and well-being go down during that time.

But as the nest starts to empty, your sense of well-being, contentment and time for intimacy go up. The U-curve begins to rise. You rediscover the person you knew early on.

Q: The book is based on interviews with 75 couples "about" their sex lives, money problems and how they handled their spouses' failings and betrayals. The participants were all older than 50 and had been married 20 years or more. What other similarities did you find in the couples?

A: They exuded well-being, contentment. The biggest thing I saw in every couple, including couples whose sex life wasn't that thrilling, was that they had a huge amount of affection and closeness. If they had conflicts, they would often end in humor or they'd say, "Let's not have this conversation." The intensity of the conflict was much lower. There are few, if any, slammed doors.

Q: What did the couples tell you about their arguments?

A: They said they argued about the same subjects that they argued about from Day 1, but there was a lowering of intensity. This could be due to biological changes in a certain brain structure known to be the seat of anxiety, fear, aggression.

Q: How do the couples forgive each other's failings and betrayals?

A: There was no across-the-board answer. Some people said they couldn't think of a big failing that the other person had.

In some cases, there had been infidelities and serious betrayals, and the couple had managed. In some cases they said their relationships had even become more sexually passionate.

People respond differently to infidelity. In my personal life, I know of one woman whose husband had been unfaithful once, and she kicked him out. In that case, the marriage had to be a dead twig that just dropped off.

Q: What insights did couples give you about their sex lives?

A: Some have about the same sex life as they always did. For other couples, there has been a slowing down. For some there is no sex life, and it's fine with them.

As they age, women are slower to lubricate and men are slower to have an erection. It has nothing to do with the fact that the other person is less sexy or likes you less. This calls for a lot more foreplay — hugging, kissing, stroking and being sensitive to the other person's needs. Plus, using a lubricant is helpful when having sex.

One man said he and his wife were having less sex than they used to, but he said he enjoyed curling up with his wife at night and kissing her lovely face in the morning.

Q: How do people get better with age?

A: As you age, it's no secret that you lose memory, vision, hearing, but there is one domain in which you probably get a lot better, and that is emotional control and emotional processing. These emotional changes promote a motivation to get the toxic people out of your life.

You're interested in being with the people closest to you and weaving a circle of positive friends and relatives — researchers call that "positivity."