IF YOU want to be president of the United States, it is a good thing to grow up tall. In the past 13 presidential elections, the taller man has won ten times. George Bush junior is a rare exception. Washington, Jefferson and Lincoln were well above average height. So was Bill Clinton (see chart). If you want to earn a lot of money, though, it seems that what matters is not how tall you are as an adult, but how tall you were as a teenager. That, at least, is the conclusion of Nicola Persico, Andrew Postlewaite and Dan Silverman, of the University of Pennsylvania.

The three economists drew their data from Britain's National Child Development Study and America's National Longitudinal Survey of Youth. They first confirmed what short people have always suspected: that it pays to be tall. In Britain, if you are white and male, an extra inch (2½cm) goes with a 1.7% increase in wages; in America, with 1.8%. The shortest quarter of the population in both countries earns, on average, about 10% less than the tallest quarter. The impact of height on earnings is smaller than the impact of race (about 15% in the United States) and sex (about 20%). But it is still significant.

That was, on the basis of earlier work, to be expected. But the researchers also observed that height differs from race and sex in a crucial respect: it changes with age. They therefore decided to test whether the earnings gap results from direct discrimination in the labour market, or from some impact during a person's formative years.

When they looked at correlations between a man's income and his height at the ages of seven, 11 and 16, as well as his adult height, Dr Persico and his colleagues found that all of the effect of height on income was actually attributable to height at the age of 16. Neither variations in increase after that age, nor differences in height earlier in childhood, had any independent correlation with income.

Why is unclear. Purely biological effects, such as genetic or nutritional differences, can probably be ruled out. They are unlikely to manifest themselves solely in someone's teens. Employers' prejudice also seems implausible: an employer is unlikely to know how tall an employee was when young. With a little statistical effort, differences in family background and schooling can be ruled out, too. The key, argue the authors, is the “social and cultural stigma” of being a short 16-year-old. Men who were relatively short youths were less likely to take part in social activities, such as athletics, than their taller peers; yet sport and clubs may boost self-esteem and inculcate valuable social skills.

The work may have broader consequences. The differences in schools and family backgrounds of tall and short youths are tiny compared with those of white and black youngsters. If a teenage sense of social exclusion influences future earnings, it may have great implications for youngsters from minority groups.