A new study has revealed why some people just can't hold their drink.

A team of scientists from the University of Helsinkin conducted a study of a gene responsible for a receptor protein in the brain. The scientists discovered that a genetic mutation is linked with impulsivity, especially when drunk.

The researchers have said that this could account for why some people cannot "hold their drink" and are prone to erratic behaviour, even after a relatively small amount of alcohol.

“The results also indicate that persons with this mutation are more impulsive by nature even when sober,” said Roope Tikkanen, a psychiatrist from the University.

So there you have it. That time we jumped into bed naked beside our in-laws wasn't because we're lightweights. It's because we have genetic mutation.

Get out of jail free card folks!