Q.

I gave my 81-year-old brother a birthday present recently. As he opened it, a brief smile crossed his face. It was funny to me because I recalled that, when he was a kid, he would tear open presents and jump out of his skin. Does enthusiasm wane as we age?

A.

I wouldn’t describe your brother as someone who has lost his enthusiasm. I would say he is just less excitable, mellower. There is scientific evidence that we do chill out as we get older.

I think that’s probably a good thing. Being at ease has become more important to me than being wildly entertained. I like sitting in a rocking chair more than I used to.

Dr. Karen Faith Berman and other scientists at the National Institute of Mental Health conducted a study about this subject. The study appeared in The Proceedings of the National Academy of Sciences.

The researchers found evidence that older brains don’t respond to rewards as strongly as younger brains do. The main difference was the way older brains responded to dopamine, a neurotransmitter or messenger.

As a chemical messenger, dopamine is similar to adrenaline. Dopamine affects brain processes that control movement, emotional response, and the ability to experience pleasure and pain. Regulation of dopamine plays a crucial role in our mental and physical health.

Neurons — or nerve cells — containing dopamine are clustered in the midbrain area. In Parkinson’s disease, the dopamine-transmitting neurons in this area die. As a result, the brains of people with Parkinson’s disease contain almost no dopamine.

Dr. Berman said the results of her study “may explain anecdotal evidence that older people are mellower, that they may not get the same highs from certain experiences, but they may not get the same lows, either.”

The researchers performed brain scans on two groups — one in their 60s, the other in their 20s — as they played a virtual slot machine on a computer.

The researchers found differences between the groups when they won at the slot machine and when they just anticipated winning. The researchers said that the differences were seen in how much dopamine was produced, which parts of the brain responded to it and how much they responded.

When a reward was anticipated, researchers said, three parts of a reward center in the brain lighted up in the younger group, but only one in the older group.

Does this mean older people are not as happy as younger ones? There’s evidence that the opposite is true. In a recent column, I reported on the phenomenon.

A Gallup telephone poll of 340,000 people across the nation showed that happiness comes with age.

The survey showed that people start out at age 18 feeling pretty good. However, they feel progressively worse until they hit 50. At that point, people begin getting happier as they age. By the time they are 85, they are even more satisfied with themselves than they were at 18.

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