The How and Why of Sex Differences

Sexually dimorphic psychological adaptations

Wrong Question: Is It Nature or Nurture?

Right question: Is it a sexually dimorphic psychological adaptation?

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Sometimes asking the wrong question will lead you down a dead-end path. A road to nowhere. And, until you reverse direction, back up, and ask the right question, you will be stuck with a flat tire on that road.

Some social scientists apparently still haven't heard the news about the nature vs. nurture debate. It is over. Good riddance. It was a dead-end path to nowhere.

The wrong question to ask is this: "Is it due to nature or nurture?" It is like asking: Is the area of a rectangle caused more by its height or width? How much of a cake is due to the cooking or the ingredients? Which roads will get me from Los Angeles to New Zealand?

Historically, cultural determinists have assumed that the brain is pretty much a blank slate, culture writes on that blank slate, and, for the most part, biology can be pretty much ignored when explaining behavior (I have met quite a few cultural determinists). On the other hand, if one believed that the environment can be safely ignored, they might be called a genetic determinnist (I have really never met one). Interactionists believe that a sophisticated understanding of behavior requires an understanding of both biological and cultural causality (increasingly, this "integrated model" is the perspective of most social scientists).

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Nature vs. Nurture

Cultural determinists, after observing a difference between the sexes on some trait, may just assume that it is due to culture. They ask: "What is it about our culture that causes this difference?" This assumes that it is different in other cultures. This perspective has been termed "The Standard Social Science Model" (SSSM), because, well, historically it has been a pretty standard way of looking at behavior, and at sex differences. And, unfortunately, most feminist theories on the origins of sex differences made the mistake of relying on this faulty model.

Cultural determinism is still pervasive in popular culture. Notice how pleasant to the ear this sentence prefix sounds: "In our culture, males are taller than females." It feels good to say it, because it makes you feel free. In fact, it sounds so good it almost seems true. Heck, maybe there are some cultures where females are taller than males. That would be a good thing, right? Especially for coed basketball teams.

Of course, it isn't true. Height is a sexually dimorphic (different) physical adaptation that is manifest in cultures around the world.

And here is where many social scientists got stuck: they pretty much ignored the possibility that there may also be sexually dimorphic psychological adaptations. In our zeal to establish that males and females should have equal opportunities and rights under the law, we were a tad too eager to extend the notion of equal to mean that the sexes were the same. That is, we pretty much assumed that the sexes' psychological adaptations were monomorphic, that the brains of the two sexes were identical. They aren't.

Behavior is always a complex interaction of both nature and nurture, never one or the other. Many social scientists now understand this, but there are still a few laggards who still don't get it. Until they get it, progress in the social sciences, and understanding the differences between the sexes, will be hindered.

Ok. "Is it nature or nurture?" We know this is the wrong question. So, what is the right question?

There are several right questions to ask about the interaction of nature and nurture in causing behavior.

Question 1. Is the trait an evolved psychological adaptation, a byproduct of an adaptation, or random variation?

By analogy with the body, think: The umbilical cord (adaptation), the belly button (byproduct of an adaptation), and convex or concave belly button shape (random variation with respect to "innies" or "outies").

For psychological traits, think: The ability of toddlers to learn to talk virtually without training (psychological adaptation), the ability to learn to read and write (a byproduct of adaptations designed for other purposes -- our ancestors did not read or write), or random variation (within-sex variations in vocal pitch).

Traits as Adaptations, By-products, or Random Noise

Question 2. If it is a psychological adaptation, is it generally obligate (relatively robust in the face of typical environmental variation) or facultative (sensitive to typical environmental variation)?

The sweet taste of sugar and the pain of hitting your knee against concrete are the result of pretty obligate adaptations -- typical environmental variability during development does not affect them too much.

On the other hand, one's adult attachment style seems particularly sensitive to early childhood experiences. The adaptation to tan is conditional to exposure to intense sunlight. So these are examples of more facultative adaptations (how the adaptation is expressed is conditional based on typical variations in environmental input).

Question 3. If the psychological adaptation is generally facultative, what specific types of environmental inputs influence it, and how?

For example, what specific types of early childhood experiences are likely to produce an anxious, rather than an avoidant or secure, adult attachment style?

 

Now, on to psychological sex differences. What is the right question to ask next specifically with respect to differences between the sexes?

The key question is this:

Question 4. Is the psychological adaptation sexually dimorphic? That is, is the psychological adaptation the same in both sexes (monomorphic) or somewhat different in the two sexes (dimorphic)?

Let's explore some of these distinctions in more detail. First, focus on a couple of physical traits that are literally easy to see: height and hair length.

The first instinct of a cultural determinist might be to assume that the sex difference in height is due to cultural factors. Maybe we encourage males in our culture to exercise more, eat more, and maybe eat more meat than salad. Maybe that results in the height difference. And, maybe it is different in other cultures.

But it isn't different. Why? Because the sex difference in height is due to an evolved sexually dimorphic adaptation. Given typical environmental variation, and across cultures around the world, males are generally taller than females. Sure, if you wanted to do an experiment, I guess we could starve males and put them in a closet, and they would end up on average shorter than females. But, given typical enviornmental variability, height is a generally obligate adaptation. And, a sexually dimorphic one. Why height evolved to be a sexually dimorphic trait is an interesting question about ultimate causality. But, for now, I will leave that to a future post.

So, when can we expect to see a sex difference in a trait?

Given that traits are due to the combined, interactive effects of both nature and nurture, to answer this question, we need to know several things:

Is the sex difference related to a trait that is an evolved adaptation, a byproduct of an adaptation, or random variation?

If the trait is an adaptation, the next questions to ask are:

Is the adaptation sexually monomorphic or dimorphic?
Is the adaptation generally obligate or facultative?
Is the cultural sex role socialization related to the trait monomorphic (males and females are generally treated the same with respect to this trait) or dimorphic (they are generally treated differently)?
If the socialization is dimorphic, we need to know if the differential socialization generally tends to either increase or decrease the magnitude of a sex difference.

Example: Height

Height is a fairly obligate, sexually dimorphic trait, and the cultural variables that might influence the expression of this trait are fairly monomorphic (that is, males and females both get enough food and exercise). Given the above, it is not surprising to see an average group difference in height between the sexes.

Height: A Sexually Deimorphic Adaptation


On the other hand, I presume that head hair length, if hair was allowed to grow out to its full length, would be pretty much the same for both sexes. That is, it is not a sexually dimorphic adaptation. However, the sex role socialization in most cultures around the world tends to be dimorphic -- the hair length of females is generally longer than that of males. Here we see a sex difference due to sex role dimorphism while the underlying adaptation is monomorphic.

Example: Hair Length

Now, why is the hair length cultural dimorphism mostly in the same direction around the world -- females with longer hair than males? Typically when we see a non-random cultural pattern such as this, we would expect that such cultural universals would be distal manifestations of an underlying psychological adaptation; in this case, a sexually dimorphic one. But here, that does not appear to be the case. Might this cultural trend be a byproduct of a sexually dimrophic adaptation(s)? If so, which one(s)? I will leave that intriguing question to the reader, for now.

A Sexually Monomorphic Trait: Head Hair Length (if left uncut)

We have been talking about sexually dimorphic physical adaptations. What about psychological ones? Let's examine food preferences (presumably a sexually monomorphic adaptation) and the propensity to engage in physical risk-taking (presumably a sexually dimorphic adaptation, males > females).

Sex differences and the interaction of nature and nurture.

In this table we can see the full range of possibilities given the interaction of nature and nurture.

Interactions Between Mono- or Di-morphic Adaptations and Socialization

If food preferences are a sexually monomorphic trait, and sex role socialization is fairly monomorphic as well in most cultures, we can expect to see no significant sex differences on this trait. Physical risk-taking appears to be a sexually dimorphic adaptation, and the cultural socialization is dimorphic as well (in the direction of increasing the sex difference), so we can expect to see significantly more males than females taking physical risks.  Note how looking at these variabiles similtaneously helps us to understand sex differences.  And, if there are sex differences that we may wish to reduce, such as greater male physical aggression, a sophisticated  understanding of this sexually dimorphic psychological adaptation will put us in a better position to do so.

Before, when we looked at sex differences from the lens of cultural determinism, we were not getting the entire picture. If we instead look at the interaction of both nature and nurture, and ask the right set of questions, the picture becomes far more clear. We can start moving forward toward a more sophisticated understanding of sex differences.

Asking the right set of questions gets us moving.  We are back on the road again.

 

Additional reading:

     See this post by PT Blogger John Johnson.

 

Copyright © Michael Mills. All rights reserved.

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Michael Mills, Ph.D. is an associate professor of psychology at Loyola Marymount University in Los Angeles.

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