There seems to be a belief among some in the comment section that genetic effects are not (primarily) additive. By additive, we mean that by adding the gene, we will add to your phenotype; for example Steve Hsu estimates that replacing 100 negative height variants in your genome with 100 positive ones will add a few inches of height. That’s not to say that everyone with a given genotype will have the same phenotype regardless of environment, but rather it’s to say that good environments combine with good genes in an additive way; like a rising tide that lifts all boats without changing their relative heights.

However some are obsessed with Gene X environment interactions. That is gene A might add 1 inch if you’re in China but it will subtract 1 inch if you’re in Brazil. I don’t deny that such interactions exist. For example the Tarahumara of the Mexican highlands had very little exposure to agriculture and thus never evolved an ability to process refined carbs. So in their native lands, their genetics adds very little fat compared to whites reared in the same environment, but when they live in the United States, it adds hundreds of pounds of fat compared to American whites. So it’s not a case of a rising tide lifting all boats without changing their relative heights. In this case the rising tide (U.S. diet) lifts Tarahumara weight several times more than it lifts white weight. Tarahumara go from equal or maybe even leaner than whites when both races are raised in the wild, to several times fatter when both races are raised in a modern industrial society.

There are also gene X gene interactions (epistasis) where the effect of a given gene depends on other genes it its nexus. An obvious example is eye colour where having an allele for blue eyes only causes blue eyes when combined with another allele for blue eyes, but results in brown eyes when combined with a brown eyed allele.

These examples notwithstanding, many human traits, especially complex polygenic traits, are overwhelmingly additive. An obvious example is black people have darker skin and kinkier hair than white people, regardless of whether the two races are raised in America, Europe, Africa, China, or even Mars! There is no genes X environment interactions that alters that ranking.

People with Y chromosomes (on average) grow up to be much taller, stronger, faster, and hairier than women, regardless of whether they’re raised in India or Britain, so no gene X environment interaction. And regardless of whether they’re South Asian, white, Bantu, Pygmy, Dutch or Downs syndrome, so no gene X gene interaction either.

During the 20th century, the heights of both men and women increased dramatically all over the developed World, but that man > woman height gap remained similar. Again, the Y chromosome is an additive constant independent of what the environment adds. Similarly, ethnic differences in IQ have held constant in the United States over the 20th century, even though the average IQ of all races has increased quite a bit (see the Flynn effect).

People with a third chromosome on the 21st pair tend to be 50 IQ points lower and many inches shorter than their normal peers, regardless of race or continent, so again no interactions. Only additive independent effects.

In fact so independent are the effects of certain genes that they have similar effects on genomes and environments as disparate as humans living in the city and mice running through the grass. The FOXP2 gene is associated with language in humans. Humans with impaired FOXP2 genes struggle with words, syllables and sounds.

So what happens when you put the human version of FOXP2 in mice?

“There seems to be a change in vocalization — they squeak in a different way,” observed Nobel Prize winning geneticist Svante Paabo.

If the same gene can add vocalization skills to creatures separated by 65 million years of evolution, don’t tell me genes don’t have additive effects.

In similar experiments, scientists took the glowing gene from a species of jellyfish living in the ocean.

They found it had the same independent effect on everything from sheeps and rabbits living in fields to monkeys swinging from trees. Can’t wait for the first glowing person!

If genes can add the same effects across wildly different genomes in completely different environments, then surely they can be additive across different kinds of people living in our increasingly homogeneous global village.