The study found that if a girl became a teenage mother, the probability of her nearest younger sister doing the same increased from a one-in-five chance to two in five.

Higher education levels and a more affluent family background have tended to lower rates of teenage pregnancy in developed countries.

But Professor Carol Propper, who led the study, said: ‘These findings provide strong evidence that the contagious effect of teen motherhood in siblings is larger than the general effect of being better educated.