Sunday, March 22, 2009

Darwin on Instincts and the Expression of Emotions


In Chapter VII of the Origin of Species, Darwin proposed that instincts were behavioral adaptations that had evolved by natural selection and sexual selection. Darwin provided many examples of instinctive behaviors in animals, and suggested how such behaviors could have evolved. In particular, he proposed that animal social behavior was the result of natural selection acting at the level of “families”, rather than individuals.

In his later book, On the Expression of Emotions in Men and Animals, Darwin elaborated on the idea that behaviors are evolutionary adaptations that have evolved by natural and sexual selection. He explained the roles that emotions play in the biology of animals, and extended those explanations to humans. He argued that emotions are essentially biological processes analogous to other physiological adaptations, and that the methods by which they can be studied are similar to those by which any other inherited trait can be scientifically analyzed.


Near the end of the previous chapter, I mentioned that Darwin proposed in the Origin of Species, that “instincts” were behavioral adaptations that had evolved by natural and sexual selection.

In a chapter in the Origin entitled “Instincts,” Darwin explored this idea in the light of his overall theory of evolution by natural selection. In particular, like many of his contemporaries Darwin was fascinated by the behavior of social insects, especially ants and bees. He pointed out that their highly specialized behavior and mode of reproduction posed a serious problem for his theory, a problem that he needed very much to solve.

Darwin on Instincts

In the chapter on instincts, Darwin was very careful to distinguish between the evolution of intelligence and the evolution of instincts. In the Origin, Darwin did not speculate about the evolution of intelligence at all, but rather confined his discussion to the behavior of non-human animals. However, he did make it clear that he believed that there were strong analogies between some of the instinctive behaviors of non-human animals and similar behaviors in humans.

Darwin initially avoided defining “instincts” directly. Instead, he provided multiple examples of the kinds of behaviors he was referring to when he used the term “instinct.” In other words, he used the kind of functional analysis that I described in the previous chapter. We will see this technique being used over and over again, not only in evolutionary biology as a whole, but especially in evolutionary psychology.

The examples of instincts cited by Darwin in the Origin have the following properties:

1. Instincts are not acquired (i.e. learned) via experience.

2. On the contrary, instincts can be performed by individuals who have never learned how to perform them, nor experienced the same set of stimuli before in their lives.

3. In particular, instinctive behaviors can be elicited from animals that have been raised in isolation since birth (or since hatching, as often the animals being tested were birds).

4. Instincts are stereotyped. That is, they are performed in very much the same way every time, both by the same individual at different times and by most of the members of a given species (i.e. they can be referred to as pan-specific behaviors).

5. Furthermore (and in contrast with many human behaviors), instinctive behaviors do not seem to require judgment or reason on the part of the individuals performing them.

6. By implication, this means that instincts are essentially unconscious; that is, they are not the result of conscious deliberation or intentions.

Darwin also took great pains to distinguish between “instincts” and “habits.” He pointed out that habits are stereotyped behaviors that are acquired during an individual’s lifetime, usually by constant repetition. However, Darwin also believed that some habits could be inherited, especially as the result of use and disuse. In that respect, Darwin clearly believed in the possibility that evolution could proceed by the inheritance of acquired characteristics, a theory proposed a half century earlier by the Frenchman, Jean-Baptiste Lamarck. If he were referring to anatomical or physiological characteristics, we would be safe in rejecting Darwin’s assertion that acquired traits such as habits can be inherited. As August Weismann and others showed, acquired anatomical characteristics (such as missing tails in mice, chopped off by the experimenter) cannot be inherited from parents to offspring.

However, some behaviors are learned from other individuals, and not just from parents to offspring. To the extent that a behavior can be learned or modified during an individual’s lifetime that behavior is essentially an acquired trait that can be passed on (i.e. “inherited”). Some behaviors, in other words, follow the rules of Lamarckian evolution. As we will see, there is evidence that we inherit (via Darwinian mechanisms) the tendency to learn certain behaviors (via Lamarckian mechanisms), and that we learn such behaviors surprisingly easily.

In the Origin, Darwin focused most of his attention on the instincts of “lower” animals, especially the social insects. One of the reasons for this was that such behaviors could be explained without including either consciousness or intelligence. Furthermore, Darwin took pains to show that instincts have many of the characteristics of evolutionary adaptations:

1. Although they are pan-specific, most instincts are quite variable, both within and between individuals.

2. Like other adaptations, the instincts that are present in a population (or species) apparently change slowly and gradually over time.

3. Instincts provide a benefit primarily to the individuals performing them, and especially not to the members of other species.

4. Instincts, like other adaptations, are not perfect. Rather, they are compromises that only have to function “well enough” to result in differential survival and reproduction.

5. Finally, and most importantly:
The performance of specific instincts can be causally linked to increased survival and reproduction by the individuals performing them.

Darwin also took great pains to show that many instincts are inherited virtually unchanged from parents to offspring. The best way to do this is to show that an instinctive behavior is performed correctly without any opportunity for the performer to have learned it through experience. Darwin noted that pointers (dogs used to assist in hunting game birds) do not need to be trained in how to “point” at their quarry. On the contrary, they need to be trained to hold still when a gun is fired, and not to maul a bird if it is shot and lands nearby. “Pointing”, in other words, is an instinctive behavior that has been bred into some hunting dogs.

The heart of Darwin’s chapter on instincts in the Origin is his explanation for the evolution of sterile castes in the social ants, bees, termites, and wasps. He started out by pointing out that if he cannot do this, it would be “fatal” to his entire theory. The reason for this is that he had previously and repeatedly asserted that traits that provide a benefit exclusively to another organism cannot possibly evolve by natural selection. This is because the other individuals (the ones receiving the benefit) would therefore increase in frequency in the population, while the individuals providing the benefit would decrease in frequency until they disappeared, leaving only those individuals who did not provide the benefit.

Darwin correctly pointed out that this problem would be most acute for the social insects, because many of them have what are known as sterile castes, such as workers, warriors, etc. These are often highly modified versions of the average ant, bee, termite, or wasp, with gigantic jaws, reduced or absent wings, etc. Furthermore (and most importantly) such specialized castes consist of individuals that are sterile: they never reproduce during their own lifetimes, but rather assist the “queen” ant, bee, termite, or wasp in reproducing.

Here, then is Darwin’s potentially fatal quandary: how can an adaptation like gigantic jaws be passed on at rates sufficient to make them more common over time if the individuals that have such traits never reproduce? Stated succinctly:
How can a sterile worker pass on the trait of sterility?

Darwin’s answer was surprisingly simple: he proposed that natural selection could act at the level of “families” (i.e. groups), rather than exclusively at the level of individuals. Darwin pointed out that the problem of the evolution of sterile castes in social insects is essentially the same as the problem of how to continue getting high quality meat from domesticated farm animals, such as beef cattle. After all, when a steer is slaughtered and cut up for meat, it can’t very well pass on the traits (thick muscles, marbled fat, etc.) that made it a superior source of beef. For that matter, a steer is already out of the evolutionary race even before it is slaughtered. “Steers” are male cattle that have been castrated (which makes them fatter and more placid). Like the members of a sterile caste of social insects, steers are sterile, and therefore cannot possibly pass on to their offspring the characteristics that make them so valuable as a source of food.

So, Darwin asked, how do we continue to obtain high-quality meat from animals that are castrated and then slaughtered, rather than being bred? The answer is, we breed their closest relatives, who presumably carry the same genetic traits that made their slaughtered relatives so valuable. In other words, we select for a closely related group of individuals, who can therefore evolve particular desirable traits, without all of them necessarily surviving and reproducing.

In the same way, the reproductive members of a hive of social insects (i.e. the “queens” and “drones”) can increase in relative frequency because of the attributes and actions of their sterile relatives (i.e. the “workers” and “warriors”). Darwin even went so far as to indirectly suggest that this is how the cells of a multicellular organism can become specialized for particular traits, even though only the germ cells (i.e. the eggs and sperm cells) reproduce. In both the case of social insects and the case of the cells of a multicellular organism, selection is considered to be operating at the level of groups (Darwin referred explicitly to “families”), rather than strictly at the level of individuals.

As we will see in later chapters, the question of what level natural selection operates is of crucial importance to any theory of the evolution of social behavior. It is often asserted that “Darwinian” natural selection can only operate at the level of individual organisms. In other words, populations (i.e. groups) of organisms are what change over time – that is, populations evolve. However, the process by which they evolve – natural selection – occurs when certain individuals with particular traits survive and reproduce more often than others – that is, individuals are selected. As is often asserted by evolutionary biologists:
Populations evolve, whereas individuals are selected.

However, even Darwin himself argued otherwise: that in the social insects (and, by implication, in social animals in general) natural selection can operate at the level of groups as well as at the level of individuals. In later chapters we will explore these ideas in more detail, and will see that there is a resolution to this seeming paradox. As we will see, that resolution is founded on the idea that ultimately genes are what produce the traits of individual organisms. Therefore, natural selection at the level of genes (rather than individuals) can be used as the explanation for the evolution of many social behaviors, especially the evolution of sterile castes in the social insects.

Darwin on the Expression of Emotions in Men and Animals

In one of his last books, The Expression of the Emotions in Men and Animals (published in 1872) Darwin explored the evolution of behavior in more detail. In it, he elaborated on the idea that behaviors are evolutionary adaptations that have evolved by natural and sexual selection. In particular, he explained the roles that emotions play in the biology of animals, and extended those explanations to humans. He argued that emotions are essentially biological processes analogous to other anatomical and physiological adaptations, and that the methods by which they can be studied are similar to those by which any other inherited trait can be scientifically analyzed.

Darwin used the new technology of photography to illustrate how facial expressions in humans are similar to the facial expressions of other animals in similar situations, such as anger and fear. For example, he argued that the similarities between the “sneer” of a contemptuous human and the raised lips and exposed canine teeth of a snarling dog are not accidental. Both communicate specific states of emotion, and therefore both have adaptive value in the context of social interactions within groups of animals (and, by implication, human social groups). Darwin was essentially making the same argument that William James was to make less than two decades later in his landmark text, Principles of Psychology; that humans have more instincts than other animals, rather than fewer.

In the last chapter of The Expression of the Emotions in Men and Animals, Darwin summarized what he believed to be the three mechanisms by which the expression of emotions in animals and humans have evolved. As he also did in the Origin of Species, Darwin asserted that some emotional expressions are the result of repeated habit that had eventually become hereditary. Again, this is essentially an argument for Lamarckian evolution by means of the inheritance of acquired characteristics. As I have pointed out before, while anatomical and physiological characteristics cannot be passed on in this way, it is possible for behaviors to be acquired and inherited by Lamarckian mechanisms. This would especially be the case if what were inherited (in strictly Darwinian terms) were the tendency to learn a particular behavior in a particular way.

Darwin also pointed out that the principle of “antithesis” was central to the communication of emotions and intentions. For example, the facial expressions and body postures that express dominance –

erect hair, forward-pointing ears and directed gaze, stiff and erect posture, etc. – are the antithesis of those that express submission –

flattened hair and ears, averted gaze, and downward-curled posture. As we will see later, the expression of emotions and intentions, and the ability to detect these, are central to any understanding of the evolution of human behavior.

Throughout most of the Expression of Emotions, Darwin approached the subject of emotional expression as if it were a specialized sub-discipline of anatomy and physiology. For example, Darwin began with a detailed examination of the musculature of the human face

showing the various muscles that, when contracted, produce the facial expressions that we associate with particular emotions. Darwin referred to the work of several anatomists and physiologists who had studied the muscles and mechanisms of emotional expression in humans, pointing out the essentially physiological nature of these processes.

He then compared the facial expressions of various animals,

including dogs, cats, and the crested macaque (an Indonesian monkey), showing the various similarities in expression of emotion.

From his analysis of the expression of emotions in non-human animals, Darwin then went on to examine the expression of emotions in humans. Here, he used the newly developed technology of photography to great effect, presenting photographs of children and adults expressing anxiety, grief, dejection, despair, joy, love, devotion, ill-temper, sulkiness, determination, hatred, anger, disdain, contempt, disgust, quilt, pride, helplessness, patience, puzzlement, surprise, astonishment, fear, horror, shame, shyness, modesty, and included a physiological analysis of blushing.

He compared natural expressions with facial expressions produced using electrodes attached to the facial muscles of volunteers.

Reading Darwin’s book, it’s clear that he thought of emotions as physiological responses to environmental stimuli. The bulk of the book is taken up with the anatomy and physiology of facial expression via muscle contraction, powered by blood flow through the circulatory system. His intent was and is clear; to show that the capacity for the expression of emotions in animals and people is an evolutionary adaptation, based on what could best be described as physiological processes.

Finally, Darwin asserted that much of animal and human behavior is the result of “the direct action of the excited nervous system…independent of the will, and independently…of habit.” In essence, this is an argument against the idea that human behavior is the result of “free will” or conscious intent. Freudian psychology caused a firestorm of controversy in western culture because Freud also suggested that most of human behavior was motivated by drives that were largely unconscious, and therefore not the result of “free will.” Despite a century of research into animal and human behavior, this idea – that our actions are largely not the result of “free will” – is still hugely controversial, even among evolutionary biologists.

In the last chapter of The Expression of the Emotions in Men and Animals, Darwin concluded “[t]hat the chief expressive actions, exhibited by man and by the lower animals, are now innate or inherited – that is, have not been learnt by the individual – is admitted by every one.” [Emphasis added] This conclusion would seem to have laid the groundwork for the science we now call evolutionary psychology. Indeed, had evolutionary biologists followed up on Darwin’s suggestions, it is quite possible that a detailed science of evolutionary psychology might have evolved in the first half of the 20th century.

However, just the opposite happened – instead of guiding the science of human behavior, Darwin’s theory of evolution by natural selection was eclipsed by a view of human nature that completely rejected any possibility that evolution (or even biology) played any significant role in human psychology. At the same time, a new discipline within the science of evolutionary biology was founded that eventually made it possible for evolutionary psychology to make a new start in the science of human nature. That discipline – theoretical population genetics – will be the subject of the last chapter in this first part of our course on evolutionary psychology.

Essential Reading:

Darwin, C. (1859) On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, 1st ed., Chapter VII: Instinct. John Murray. Available online here.

Darwin, C. (1872) On the Expression of Emotions in Men and Animals. John Murray. Available online here.

Supplemental Reading:

James, William (1890) Principles of Psychology. Henry Holt. Available online here.

Questions to Consider:

1. Can instinctive behaviors have learned components, and vice versa?

2. Why did Darwin focus on the expression of emotions when he analyzed the evolution of behavior in humans and other animals?


As always, comments, criticisms, and suggestions are warmly welcomed!


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