My next two examples illustrate how genes and environment interact to affect behavior.

We mentioned that enzymes are proteins, and the level of a particular enzyme present is ultimately determined by the activity of the gene for that protein. Let’s say the particular protein is “tryptophan hydroxylase”. Why is TH important? It is the enzyme that produces serotonin, a neurotransmitter from the amino acid tryptophan. What effect does serotonin have on behavior? We know the effects that serotonin has on behavior when we see the effects of serotonin enhancing drugs on behavior (like Prozac).

Now, how can the environment influence something like serotonin levels? It seems pretty clear that the amount of serotonin present should be a direct result of the activity of our genes, right??? Not only is the enzyme for producing serotonin a protein, so is the receptor that serotonin binds to! But, does anyone know what tryptophan comes from? Our diets! So if we are deficient in tryptophan, it doesn’t matter how much gene activity we have--- we’re going to have less serotonin no matter what our genes are doing!

Lastly, and I think this is really the most revealing finding regarding the interaction: a study investigated a cohort of a thousand men living in New Zealand…followed from birth so detailed records were made of their rearing environment from the time of birth. The men were all white, and had little variation in class or wealth—precisely the kinds of conditions that allow us to make conclusions about heritability. The researchers were interested in poor outcomes as adults—what were the characteristics (genetically and otherwise) of those members of the cohort that turned out to be criminals?

In the cohort, they identified those that had been maltreated as children—some 8% of the sample had been severely abused, 28% had probably been maltreated—there was a higher percentage of criminal behavior, getting into trouble, showing antisocial personalities in this subset of individuals. This is where most other studies would have ended and concluded long-term negative outcomes associated with childhood maltreatment…but they also looked at the genes of their subjects—they were particularly interested in particular variants of the gene controlling the amount of an enzyme produced. The enzyme is MAO—the enzyme that breaks down serotonin (and other neurotransmitters) and ultimately determines the level of neurotransmitter present. But it wasn’t the MAO gene they were looking for. Upstream of the MAO gene (on the same chromosome), is a promoter region—a gene that turns on the MAO gene—the promoter comes in 4 different variants (specifically, what varies is the number of sequence repeats of a particular region of the promoter)—2 of the variants cause low activity of the MAO gene; 2 of the variants cause high activity of the MAO gene. (Click here for the link to the Moffitt study (study first author is Caspi…the correct citation is Caspi et al. 2002))

Violent behavior occurred most frequently in those who had BOTH been maltreated as children and had one of the low-activity variants of the gene. Low-active maltreated men committed 4x their share of violent behaviors.

It is not enough to experience maltreatment; you must also have the low-active gene. Likewise, it is not enough to have the low-active gene; you must also be maltreated.

The last thing I want to cover in relation to this topic, then is the evolution of behavior, and the relationship among primate species. Now we turn our discussion to those things that make us all alike, rather than those contributors to our differences. In many cases in this course, we will be looking to the behavior of other species of animals, particularly other primate species, to demonstrate how human behaviors evolved, or came about. This approach is appropriate because we as human beings shared a common ancestor with other primate species—this is what Darwin postulated in the mid 19^th Century. When considering that Darwin’s theory did not gain widespread acceptance in his lifetime, this is an awfully short period of time in the history of our species that we have had an accurate version of the origin of our species.

Evolution is really about speciation—how different species arise from common ancestry. When we talk about relatedness of different animal species, we’re really talking about time since we shared a common ancestor. Your siblings are closely related to because you shared a common ancestor in the last generation (your parents). Your cousins are less closely related, because your common ancestor was 2 generations ago (your grandparents). All human beings shared a common ancestor roughly 1000 generations ago. This same logic can be applied to all living things on Earth, who shared a common ancestor 4 billion years ago. The basis of evolution by natural selection is that in the course of replicating, mutations occur. Darwin had no idea how likely replication errors were to occur, no-one did until DNA was understood…now it’s clear that changes in DNA sequences happen all the time during replication, so mutation is capable of driving evolutionary change. If the mutation is favorable, then those descendents who inherit it will thrive and become more highly represented in subsequent generations. If the mutation is not favorable, the mutation will die with the organism that houses it (provided it has not had time to reproduce and pass on its mutated gene). This is the basis of natural selection. Eventually, mutations will accumulate, accomplishing “speciation”…that is, the divergence of a new species. In this way, it is believed that humans diverged from the common ancestor that we shared with other non-human primates (more about this later).

In sum…evolutionary theory holds that that human beings (and all other organisms) came about through a series of gradual changes in a common ancestor. Individuals in a species exhibit variability that has some genetic basis—heritable. If those variations lead to enhanced reproductive success, those trait variants will be more highly represented in subsequent generations—eventually, variability in that trait will disappear, as all members of the species come to possess that trait. We can think of this as the mechanism that underlies the human universal psychology that we discussed earlier in the semester, and what I hope to do is to demonstrate that some universal psychological traits are not even specific to humans…they may have been something shared by our closest evolutionary relatives.