Scientists have uncovered clues about what happens in the brain to make some people "over-friendly".
The researchers scanned the brains of people in the study
US National Institute of Mental Health experts looked at differences in the brains of people with an abnormality which makes them highly sociable.
Researchers used scans to identify areas which failed to work properly when they saw frightening faces.
In Nature Neuroscience, they say this could give clues for understanding social disorders in others.
People with the genetic condition Williams Syndrome lack around 21 genes on chromosome seven.
Their lack of fear means they will impulsively engage in social situations, even with strangers.
But they often have heightened anxiety about non-human fears, such as spiders or heights.
The condition affects around one in 25,000 people.
The US team focused on the amygdala, an almond-shaped structure deep in the brain which has been thought to help regulate social behaviour.
fMRI (functional Magnetic Resonance Imaging) scans was used to study the brains of 13 healthy volunteers and 13 with Williams Syndrome.
All were shown pictures of angry or scary faces.
In healthy brains, seeing such images would provoke a strong response in the amygdala.
However the fMRI scans showed far less activity in those of people with Williams Syndrome.
Study participants were then shown pictures of threatening scenes, such as plane crashes, which did not have any people or faces in them.
The amygdala response was seen to be abnormally increased in participants with Williams Syndrome.
The researchers also identified three key areas of the prefrontal cortex, located in the front part of the brain, which did not behave normally in people with the syndrome.
They were the dorsolateral area - linked to establishing and maintaining social goals governing an interaction; the medial area - associated with empathy and regulating negative emotions; and the orbitofrontal region - involved in assigning emotional values to a situation.
Thomas Insel, director of the NIMH, said: "Social interactions are central to human experience and well-being, and are adversely affected in psychiatric illness.
"This may be the first study to identify functional disturbances in a brain pathway associated with abnormal social behaviour caused by a genetic disorder."