In genetics, there is a concept known as penetrance. This concept is typically most relevant with dominant mutations that cause disease and the idea here is that not all genotypes elicit their expected phenotypes. For example, consider the phenonmena of polydactyly in humans. This is where an individual has extra fingers and/or toes. Since this trait is caused by a dominant mutation, you would expect that anyone with the dominant allele would have this trait. Yet this is not always true. The concept of penetrance comes into play when we estimate how many with a particular genotype express the trait. For example, if 90 out of 100 people who are heterozygous have the trait, we’d say the trait is 90% penetrant.

So why is it that many traits show less than 100% penetrance? Two factors come into play – the genetic background and the environment. Whether or not a particular allele at a specific locus is expressed can be a function of the expression of other alleles at other loci. Thus, without the right genetic context, a particular genotype may not be expressed. As for environment, it is well known that it can work in conjunction with a genotype to determine whether a particular phenotype is seen. This means that certain traits will be expressed only in the right environmental context.

I mention all of this simply because it makes for a nice metaphor in understanding how humans believe. Many people share the naïve notion that a powerful argument for X should elicit belief X. If someone is thus presented with argument X, yet fails to adopt belief X, that person is then viewed negatively (i.e., they are stupid, deluded, or dishonest).

But let’s assume the argument is the allele (genotype) and the belief is the trait (phenotype). Whether the argument leads to belief depends on the context of background beliefs and experience that already exist (akin to genetic background) and the social setting (the environment). The argument for X may fail to elicit belief X simply because of incomplete penetrance. In such cases, the power of the argument for X is dependent on the context of other beliefs and knowledge and the way belief X plays out in social reality.

Such incomplete penetrance is not stupidity, delusion, or dishonesty. It exists as a function of the Complexity of Belief. We not only believe differently, but we think differently. Thus, an important lesson in life is to realize that other people are not extensions of your self.