Some people may be genetically prone to be lonely, a new study suggested.
Being socially isolated leads to an early grave and can be as bad for you as smoking 15 cigarettes a day.
A new study of rhesus macaques showed some of the monkeys remain socially isolated for much of their lives.
This suggested their isolation is caused by a persistent trait or traits.
The researchers from the universities of Exeter, Puerto Rico and Pennsylvania believed the cause is a mixture of their genes and other factors such as age, sex and family size.
Dr Lauren Brent at Exeter’s department of psychology said: “Understanding social isolation is really important, and studying macaques might give us clues about human behaviour.
“Isolation is the latest epidemic among humans, and research has suggested it is as bad for us as smoking 15 cigarettes a day.
“Given the benefits of social integration, we need to understand why some individual animals tend to be socially isolated.”
The study published in the journal Scientific Reports measured integration by observing how much time macaques spent grooming others and being groomed – a key social behaviour for the species.
Factors that led to isolation were age, sex, social status, group size and how long a macaque had belonged to a social group.
However, the identity of an animal’s mother did not play a role, suggesting behaviour that leads to isolation is not learned from the mother.
Dr Brent said the findings suggested isolation could be partly maintained by natural selection – meaning there might be some evolutionary benefits.
The possible benefits of isolation include lower risks of disease and conflict.
The data came from 429 adult rhesus macaques at the Cayo Santiago field station in Puerto Rico, which was devastated earlier this year by Hurricane Maria.
Researchers including Dr Brent have raised more than £70,000 to repair the field station and support people living nearby.