I believe that the current understanding of the role of genetics in human emotional development is flawed. Below, I explain some of these flaws.

Summary: Heritability is a statistic used to measure the effect of genetics on traits. I believe that it is improperly measured when studying humans. If properly measured, I believe that it will support my claims regarding the role of genetics in humans as described on the Effects of Childhood Experiences page. In addition, many population-based studies of the role of genetics in human variation are flawed.

Heritability is improperly measured when studying humans.

Heritability is based on the equal environment assumption (EEA). The EEA assumes that the environment of identical (monozygotic=MZ) twins and non-identical (dizygotic=DZ) twins are equally similar. In other words, it assumes that MZ twins are treated similarly to some degree and DZ twins are treated similarly to approximately the same degree.

The environment used to determine the EEA in twin studies is measured using questionnaires given to both the parents of twins and the twins themselves that ask about how similarly the twins were dressed, how much they played together, how many teachers they had in common, whether or not they slept in the same room, how many friends they had in common, how often they socialized together, and so on.

I believe that the environment that produces our personalities is the parents' emotion as displayed primarily on their faces and in their voices when around their children (see How Personality Develops for details). This has little to do with how many teachers they had in common, whether or not they slept in the same room, how many friends they had in common, and so on. I believe that MZ twins are treated more similarly than DZ twins, as explained below.

MZ twins are more similar than DZ twins because their environment is more similar. Here's how that works:

Let's assume that at birth, DZ twins react to a situation with identical emotions. However, since their faces and eventually their voices are different, they will express those identical emotions slightly differently. Their parent, who perceives a slightly different emotion in each twin, will react slightly differently to each twin, even though the twins are expressing identical underlying emotions. As a result of the parents' slightly different response to each twin, each DZ twin will incorporate a slightly different emotional response to that situation (see the How Personality Develops page for details). Consequently, the next time a similar situation arises, the DZ twins will react internally with slightly different internal emotions. Those different internal emotions will be expressed even more differently because of their different faces and voices. Their parent will then react more differently to each twin which will cause the twins to incorporate even more different emotional responses to that situation. As this process repeats, the DZ twins' emotional responses will diverge.

By contrast, when MZ twins react to a situation with identical emotions, they express those identical emotions almost identically. That's because their faces and their voices are nearly identical, especially compared to DZ twins. Their parent, who perceives nearly identical emotions in the twins, will react more similarly to each of them than they do to DZ twins who express identical emotions differently. As a result of their parents' similar responses, the MZ twins will incorporate similar emotional responses to that situation, especially compared to DZ twins. Consequently, in subsequent similar situations, MZ twins will react internally with similar emotions, especially compared to DZ twins. Those similar internal emotions will be expressed similarly and the parent will respond similarly, especially compared to DZ twins. As this process repeats, the MZ twins will incorporate relatively similar emotional responses, especially compared to the diverging responses of DZ twins.

Many of the flaws in population studies that study the genetic basis of human traits are thoroughly explained in Jay Joseph's book "The Gene Illusion" (2004 by Algora Publishing). Below are two of the many problems he discusses in his book:

Problems with studies of twins "reared apart" include: 1) The twins were often not adopted apart at birth, so they had similar environments during their early, formative months and years, 2) after being adopted apart, the twins often had a significant amount of contact with each other, and 3) the twins were often raised by relatives who may share similar parenting styles with the parents.

The concordance rate of schizophrenia in identical twins prior to 1962, when the diagnostic criteria were much broader, was about 60%, while the concordance rate between 1962 and 2004 was about 21%. Consequently, researchers who include the studies before 1962 report significantly higher concordance rates than researchers who omit studies before 1962.