Photo by ximagination / 123RF

Source: Photo by ximagination / 123RF

Why some people find it easy to give up meat and others do not. When our 12-year-old daughter Katie became a vegetarian, we thought she was going through a phase. Wrong. Katie did not eat meat for the next 25 years.

Her sister Betsy, by contrast, has always been an adventurous omnivore – she has consumed worms in Thailand, cricket in Mexico and whale meat in Japan. (She says it is harsh and greasy.) The differences in our girls’ relationship with meat are especially interesting because Betsy and Katie are twins.

Why should two children with the same parents, born eight minutes apart and exposed to the same food environment from birth, have such different attitudes towards meat? Dutch researchers recently examined the relative importance of heredity and environment in the decision to consume vegetarian and in gender differences in the influence of genes on meat eating. Their results are astonishing.

Assessing the impact of nature and education on behavior

Betsy and Katie are fraternal twins. If genes play an essential role in the development of psychological traits, identical twins (who share 100 percent of their genes) should be more similar than fraternal twins like my daughters, who share, on average, 50 percent of their genes. Behavioral genetics use similarities and differences between identical and fraternal twin pairs to calculate a statistic called heritage. This is the percentage of individual differences in a trait attributed to genes.

Inheritance can range from 1 (individual differences are entirely genetic) to 0 (genes play no role in individual differences). Hundreds of studies have found that genes affect almost all of a person’s psychological traits. For example, genes account for 40-50 percent of differences in basic personality traits, approximately 40 percent in political attitudes, about 30 percent of differences in sexual orientation, and 50 percent in dog ownership. Researchers have generally found that individual changes in meat and vegetable preferences are 30-50 percent attributable to gene influence.

The Genetics of Vegetarianism Behavior

The new study by the Dutch research team is the first to examine the role of genes in vegetarianism. They studied the food preferences of 7,197 adult twins and their non-twin siblings extracted from the Central Population Register of Finland. In addition to examining the influence of genes on decisions to become vegetarian, they were interested in how exposure to meat and plant-based foods during childhood influenced adult food preferences and gender differences in the influence of genes on food preferences.

Participants completed a series of questions on their preferences for three types of meat and three types of vegetables, the frequency with which these foods were served as children, and their willingness to try two dozen new types of meat and vegetables. Finally, they were asked, “Are you a vegetarian or a vegan?”

Surprising results

Some of the results were not surprising. Men, for example, preferred meat more than women. Men were also more willing to try new types of meat, and they ate more meat when they were children.

Women preferred to eat more plants than men and were more willing to try new plant-based foods. As you might expect, participants exposed to meat more as a child liked meat more when they grew up – much like plant-based foods.

Other results were unpredictable. For example, in childhood, genes played a greater role in individual differences in meat eating in boys (31 percent genetic) than in girls (11 percent genetic). But when the participants grew to adulthood, the opposite was true. Only 26 percent of the differences between males in meat preferences were attributed to genes compared to 55 percent of the differences between females.

But the biggest surprise was the extent to which genes influenced decisions to become vegetarian or vegan. Fifteen percent of women and five percent of men identified themselves as vegetarians or vegans. (Due to their small numbers, vegans were combined with vegetarians in this analysis.)

Identical twins were much more similar in their decision to give up flesh than fraternal twins. (For statistics, “polychloric correlations” were 0.76 for identical twin couples compared to 0.38 for fraternal twins.) While math is beyond the scope of this blog, researchers estimated that 75 percent of individual differences in eating decisions or not meat was attributed to the direct and indirect influence of genes.

After all

The results of the study raise some interesting questions. Take the sexual differences. Why did the influence of environmental factors on carnivore increase and genes decrease as men (but not women) increase into adulthood? The researchers suggested that the greater importance of environmental factors in the consumption of male meat reflects gender differences in socialization processes. That makes sense to me.

However, the study’s most surprising finding was that 75 percent of the individual differences in vegetarianism were attributed to genes. By comparison, twin studies typically find that the influence of genes on individual differences in human behavior is in the 20-50 percent range. The reasons for the high degree of influence of genes on vegetarianism are unclear and numerous factors and many genes may play a role. For example, they may be due to genetic changes in taste sensitivity (see this blog post) or nutritional needs.

The most interesting idea is that decisions to give up eating meat are influenced by genetically influenced changes in moral values. Indeed, using a large sample of twins extracted from the Finish register, the Dutch research team recently reported that genes played major roles in individual differences in sexual attitudes and behaviors and in supporting or condemning recreational drug use.

Like any study, this research has its limitations. In fact, Laura Wesseldijk, one of the study’s authors, wrote to me in an email,

I think we should interpret these results carefully … Note that heritage assessment reflects the importance of genetic influences in this particular Finnish sample. It would be helpful to repeat this in other countries, hopefully in larger samples with more vegetarians (especially males)!

However, I expect the results of this study of genetic effects in Finnish twin diets to apply generally to other populations – at least in Western countries. However, I think repetition of research will reveal that the genetic component of vegetarianism is closer to 40% or 50 percent than 75 percent.

In an email, Dr. Wesseldijk told me: “The most important thing is for people to have a bigger picture of what this research shows instead of specific statistics.”

She is right. According to a recent report by Yale University researchers, only five percent of Americans are vegetarian or vegan – numbers that have been stable for decades. As shown by the great cultural differences in meat consumption per capita, biology is not the fate of eating meat.

However, research into behavioral genetics may help explain why it is so difficult for most people to give up meat and why most vegetarians and vegans eventually resume their carnivorous way.