Are you a short man or an overweight woman? If so, you may have a slight disadvantage in life compared with taller men and thinner women.

Our latest study has found evidence that men who are shorter due to their genes have lower incomes, lower levels of education, and lowlier occupations than their taller counterparts. The effect of height on socioeconomic status was much weaker in women. In contrast, women who have a higher body mass index (BMI) due to their genes have lower standards of living and household incomes. Having a higher BMI didn’t seem to have the same negative effect on men.

Don’t we know this already?

Why did we want to do this study? After all, didn’t we know that height and BMI are associated with socioeconomic status? And you can’t change your genes, so why is this study interesting?

It’s true that we have known for a long time that being short is associated with poverty, almost certainly because poor nutrition in childhood stunts growth. But the relationship between fatness and poverty is more nuanced.

In the not too distant past being thin was associated with poverty, and being overweight with wealth because people with more money were able to eat more. However, in the past few generations, in developed countries, that association has reversed. As we have moved to a world where calorie-dense food is readily and cheaply available, and life has become more sedentary, lower standards of living are associated with higher BMIs. But in this study we wanted to answer questions about causality rather than associations, which is why we turned to genetics.

You can’t change your genes

Associations between genes and human traits are likely to be cause not consequence. We can make this statement because your genes don’t change.

A disease can’t change your DNA sequence, but your DNA sequence can influence your chances of developing a disease, growing more, or your vulnerability to obesity. Once your father’s sperm has fertilised your mother’s egg, you are stuck with those two copies of the human genome and with some exceptions, such as in cancer cells, those two DNA sequences change very little during our lifetimes.

The different environments we encounter, the lifestyle choices we make, and the diseases we develop do not change the DNA sequences we inherit from our parents – to be clear, we are not discussing epigenetics here, where the environment can change how genes activate and deactivate.

Shorter men and heavier women are poorer

We used demographic and genetic data from 120,000 people (aged between 40 and 70) in the UK Biobank. The study used 400 genetic variants that are associated with height, and 70 associated with BMI, together with actual height and weight, to ask whether or not shorter stature or higher BMI could lead to lower chances in life – as measured by information the participants provided about their lives.

Having analysed the data, we found that men who were 7.5cm shorter, for no other reason than their genes, on average earned £1,500 a year less than their taller counterparts. Meanwhile, women who were 6.3kg heavier, for no other reason than their genes, on average earned £1,500 a year less than the lighter women of the same height.

It’s important to note that these are estimates and averages – short men and heavier women can, and do, succeed in life. Instead, it shows that across the population overweight women and shorter men are, on average, slightly worse off.

What are the implications?

We now need to understand the factors that lead people who are overweight or short to lower standards of living. Is the link down to low self-esteem or depression, for example? Or is it more to do with discrimination?

In a world where we are obsessed with body image, are employers biased? And do we need to pay more attention to potential unconscious biases in order not to unfairly discriminate against people who are shorter (especially men) or overweight (especially women)?

More studies are needed using data from other birth cohorts – the UK Biobank is biased towards thinner people and wealthier people because they had to actively participate in a study about health and this bias may have affected the results (that is, made the associations slightly weaker).

The study was also limited to people born between 1935 and 1971 and so the effects may no longer exist in younger adults today. It will be interesting to study the effects in young adults – it may be that the higher levels of obesity would exacerbate the problem, or it may be that society is far more accepting of fatter people and that factors such as discrimination and social esteem, if they were key to this data, are less important in younger generations.

The study provides a much needed advance in understanding a classic chicken or egg problem. But something about having a higher BMI as a woman, and shorter height as a man, does lead to being worse off in life.