As a man fathers more daughters, he will gradually be won round by their more Left-wing viewpoints.

The study, carried out by Professor Andrew Oswald from Warwick University and Dr Nattavudh Powdthavee from York University, also found that a predominance of sons can make a mother more right-wing.

The researchers even suggested that well-known Left-wing politicians and personalities owed their beliefs to the high numbers of daughters in the family.

The late John Smith, former leader of the Labour party, had three daughters.