What marriage can give you if you're really, really lucky:

A team from Stony Brook University in New York scanned the brains of couples who had been together for 20 years and compared them with those of new lovers. They found that about one in 10 of the mature couples exhibited the same chemical reactions when shown photographs of their loved ones as people commonly do in the early stages of a relationship.

Previous research suggested that the first stages of romantic love, a rollercoaster ride of mood swings and obsessions that psychologists call limerence, start to fade within 15 months. After 10 years the chemical tide has ebbed away. The scans of some of the long-term couples, however, revealed that elements of limerence mature, enabling them to enjoy what a new report calls "intensive companionship and sexual liveliness". The researchers nicknamed the couples "swans" because they have similar mental "love maps" to animals that mate for life such as swans, voles and grey foxes.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.