Researchers found women who said their natural hair colour was blonde had an average IQ score within 3 points of brunettes and those with red or black hair.

Data for the study came from the National Longitudinal Survey of Youth 1979 (NLSY79) – a national survey of people who were between 14 and 21 years old when they were first interviewed in 1979.

In 1980, participants in the NLSY79 took the Armed Forces Qualification Test, or AFQT, which is used by the Pentagon to determine the intelligence of all recruits.

The overall AFQT score is based on word knowledge, paragraph comprehension, math knowledge and arithmetic reasoning.