Falling madly in love may really make you mentally ill, according to Italian scientists.

Their research found emotional and biological similarities between people in love and those suffering from a psychiatric disorder, says a report in New Scientist magazine.

Psychiatrist Donatella Marazziti, of the University of Pisa, noticed how lovesick youngsters' one-track thoughts mirrored those of people with a mental illness called obsessive compulsive disorder (OCD).

OCD sufferers experience nagging, anxious thoughts and feel compelled to repeat tasks such as washing their hands and tidying up.

In 1990, Dr Marazziti found OCD was linked to lower levels of the brain chemical serotonin, which affects people's moods.

But she was struck by how sufferers' obsessive emotions recalled those of people newly in love, who often praise their loved ones wit and beauty for hours on end.

When she and her team compared serotonin levels between 20 lovesick Italian students and 20 people with OCD, they discovered both groups had similarly low levels of the brain chemical.

Obsession aids evolution

"It is often said that when you're in love, you're a little bit crazy," said Dr Marazziti. "That may be true."

University of California psychiatrist Hagop Akiskal, one of the research team, suggested this obsessive behaviour may be linked to evolution.

Without intense emotion, he said no-one in their "right" mind would fall in love and have children.

But the study found the first flush of love does not last as emotions settle down.

When the researchers tested the students a year later, they found their serotonin levels had returned to normal and their obsession with their partners had died down.

The search for love

Lucy Selleck, a counsellor for marriage guidance group Relate, said people can become blinded by love and make unwise decisions.

"It seems for some people the feeling of falling in love is quite addictive," she said.

"When it levels off it's a bit of a let down so they go off seeking it again and again."

But Vicki Carr, a consultant with dating agency Drawing Down the Moon, said some find the search for love frustrating.

"The people we deal with want to experience the madness of falling in love but it is not really happening."