How do you know when you are truly in love?  Scientists from Stony Brook University in New York claim they have discovered true love.

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.

The reactions of the swans to pictures of their beloved were identified on MRI brain scans as a burst of pleasure-producing dopamine more commonly seen in couples who are gripped in the first flush of lust.

“The findings go against the traditional view of romance – that it drops off sharply in the first decade – but we are sure it’s real,” said Arthur Aron, a psychologist at Stony Brook.