Photo by JD Hancock via Flickr

Some people try drugs*, others do not. Some become regular users, others do not. Some become addicted, others do not. Some recover, others do not. Why?

Much of psychology focuses on how people behave in general. As a behavior geneticist, I study what makes people different. One common way we explore differences between individuals is to compare twins.

Nature and nurture

Twin pairs essentially come in two types: identical (or monozygotic) and fraternal (or dizygotic). Identical twins share 100% of their genes, while fraternal twins share only around 50%** (the same as full-siblings). Because of these differences in genetic similarity, if identical twins are more alike than fraternal twins on a characteristic of interest (which could be almost anything, like height, personality, or schizophrenia), then that characteristic is at least partly heritable.

Of course, heritable or genetic influences make up only part of the picture. If a something were 100% heritable, identical twins would be 100% truly identical on it. This is rarely the case. For the vast, overwhelming majority of characteristics that vary among people, it is a matter of both nature and nurture, where nature and nurture are complex, sometimes overlapping, and often interacting with one another. With regard to drug addiction, twin studies find that around 40% of the variation between people is due to heritable factors, while the remaining 60% is due to environmental influences.

Problems of interpretation

The twin study approach is not without criticism. Dr. Nancy Segal recently addressed many of the common criticisms and misunderstandings in a post on her blog, so I won’t rehash those here.

The true influences on any human characteristic are bound to be incredibly complex. Measurement of potential influences is itself a major hurdle that we must overcome.

A thought experiment: describe your environment - all of it.

I'll wait.

Did you include what you had for breakfast? Your marital status? Whether your mother used any medications while pregnant with you? Your beliefs and values?

Identifying environmental influences on human behavior is a major job in and of itself. Genetics may seem like a relatively narrow, well-defined field by comparison, but biologists gain new insight into the human genome on a daily basis it seems, and the picture is only getting more complicated. Psychology and genetics are both young fields among the sciences, and attempting to combine the study of behavior with the study of genetics means we must keep in mind both the strengths and weaknesses of each field.

Some specifics of estimating heritability

The most frequently used method for estimating the relative influence of genes and environment on a characteristic is known as the “ACE model”. (This assumes that you already have access to information from a whole slew of twin pairs who were raised together. Otherwise, step 1, gather data.) In the ACE model, the sum total of differences between individuals is split into three broadly-defined sources of variation:

  • A, additive genetic effects: factors (that is, genes) that identical twins share 100% and fraternal twins share 50%**;

  • C, common or shared environmental effects: factors shared 100% between twins, which make a pair of twins more similar to one another regardless of whether they are identical or fraternal.

  • E, unique or non-shared environmental effects: factors unique to each individual, which make twins less similar regardless of whether they are identical or fraternal.

Twin researchers usually estimate the amount of individual differences that can be attributed to each of these factors using a method called Structural Equation Modeling. There are also many extensions and modifications that can be made to this model. For example, we could include non-twin relatives or explore alternative models of genetic effects (such as gene-environment interaction).

Estimating the relative influence of genetic and environmental factors within the ACE model does not require that we identify these “factors”. This is extremely useful, because it would be nearly impossible for us to accurately identify and measure all of the genes and environments that could possibly affect something like drug use and addiction within a single study. However, this means that after estimating the relative influence of genes and environments in general, there's still a lot of work to be done in identifying the specific genes and environments that make a difference.

* When I say “drug,” I mean any addictive substance: tobacco, alcohol, cocaine, prescription meds, caffeine, marijuana, cheesing, and so on. Well, not cheesing.***

** What does it mean to say fraternal twins share 50% of their genes, if more than 99% our DNA is identical across all humans?It’s true that more than 99% of your DNA is essentially indistinguishable from anyone else’s, but what that really means is that a lot of DNA is either non-functional or focused on the very basics of making an organism (like making sure your cells stick together). It's the less than 1% that varies between people that has the potential to influence our observed differences. This very small portion of DNA is therefore what we're interested in, and that's what is shared an average of 50% between fraternal twins.

*** “Cheesing” is a South Park parody, suggesting you can get high from cat urine (video link, NSFW). It is not real and you definitely should not try it at home.

Copyright 2012 Jaime Derringer

About the Author

Jaime Derringer, Ph.D.

Jaime Derringer, Ph.D., is a post-doctoral fellow at the Institute for Behavioral Genetics at the University of Colorado in Boulder.

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