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Egenis · News

Is the nature/nurture question in our DNA?

30.03.2011

Introduction

Professor Paul Griffiths takes a new look at an old argument.

Story

The phrase ‘nature or nurture’ was coined by Francis Galton in the 1870s, although the argument goes back to ancient times (writes Prof Griffiths). In the 20th century the dichotomy has tended to be dismissed by scientists as artificial, but apparently without reducing public interest in whether behaviour is due to nature or to nurture.

People did not wait for Galileo to describe projectile motion before throwing stones. They did not wait for Darwin before breeding plants and animals. They had their own ways to think about the topics that would later be called physics and biology. But intuitive understandings of physical phenomena can be at odds with scientific explanations. People who don’t study physics think that when they throw a stone they ‘add’ something like ‘impetus’ to the stone which keeps the stone moving after it leaves the hand. There has been no room for this idea in physics since Newton, but people continue to think that way. They continue to believe that moving objects contain a quantity of force and stop moving when it is exhausted. That picture makes intuitive sense in a way that the Newtonian picture (let alone the Einsteinian picture of curved space-time) does not.

People also have an intuitive picture of biology. In all cultures that have been studied, plants and animals are classified hierarchically. Sparrows are a kind of bird and birds are a kind of animal. Most reasoning about plants and animals takes place at a distinctive level of classification – the level of sparrows and foxes, oaks and foxgloves – and as if organisms of the same kind share something which cannot be directly observed: their nature. Their nature is something inherited from their parents which makes them what they are – a rat, a cat, a human being. The fact that each member of the species has a similar inner nature explains why the species has shared observable features – why cats arch their backs and hiss when threatened. The nature of an animal cannot easily be changed. No matter how much a cat is changed on the outside, even if it comes to look and behave more like a rat than a cat, its underlying nature remains pure cat.

Our research1 suggests that part of this intuitive picture of living things is the tendency to think about behaviours as innate or acquired, to divide behaviours ‘due to nature’ and behaviours ‘due to nurture’. We used descriptions of how birds learn their songs to reveal which information people depend on when deciding whether a song is innate – part of the bird’s nature – or acquired. The tendency to believe in two discrete sources of behaviour, one outside the animal and one inside - the animal’s ‘nature’ – may be part of the evolved psychological make-up of human beings. The idea of ‘human nature’ is simply the application of this general picture to human beings.

One of our most interesting findings was that the phrase which best expresses the idea of an underlying nature for young westerners is ‘in the DNA’. For example, the idea that a behaviour serves some purpose in the animal’s life is much more strongly associated with the phrase ‘in the DNA’ than with the word ‘innate’. This is hardly surprising in a culture in which a politician’s deepest convictions are their &ssquo;political DNA’ or where you can rely on a brand because ‘quality is in its DNA’. But it does have an important implication. Outside the biology department, DNA is only the current label for a very old way of thinking. The new, more scientific-sounding phrase expresses the same thought our great-great-grandparents expressed by saying that something was in the blood.

The persistence of the intuitive picture is problematic because modern genetics and developmental biology are not a straightforward vindication of that older picture. If molecular biology, in particular, is assimilated to the traditional, intuitive picture it comes out something like this: organisms get DNA from their parents. Some traits of organisms, including some behaviours, are written in the DNA. These form a universal human nature that we can expect to find in all normal human beings, or all members of some natural category such as toddlers or old men. As for the environment, every organism needs basic things like food to keep it alive, but when the environment produces a specific outcome, such as a strong stress-reaction, or obesity, those are acquired characters and not part of human nature. We can find out about the true, underlying human nature by isolating people from the interfering effects of particular environments, especially cultures. We can also find out about human nature by seeing what stays the same in all cultures.

Now contrast this to another picture, arguably much closer to the facts as we currently understand them. Organisms inherit DNA from their parents. About two percent of this is coding DNA – the 20,000 or so human genes. Most of these genes are generic and shared with many other species. Typically, many genes are involved in the production of each observable trait of the organism and each gene is involved in many traits. There is substantial variation between individual human genomes. There are often different but equivalent ways to produce the same trait. A large but currently unknown proportion of the other 98 per cent of the DNA is involved in regulating which genes are used and how they are used at each point in the life of the organism. The system is designed to work in a very specific environmental niche, which in humans includes socialization and exposure to all the factors that make up a human culture. These factors also help regulate which genes are used and how they are used.

This developmental system is designed to produce flexible outcomes depending on the circumstances in which the individual finds themselves. It also responds to cues from the parent about the likely future their offspring will face, based on the parent’s experiences. Stable features, found in most healthy individuals, are usually not stable because the underlying genes are more similar between individuals, but because the system is designed to reach the same outcome from different starting points by compensating for those differences. Stable features can also depend on very specific environment cues, as long as those cues are reliable or are made reliable by the activity of parents or other human beings.

If this is how things really work, then human nature includes not just what all humans have in common, but also many aspects of human diversity. Traits that vary can be part of human nature just as much as traits that are stable across cultures, because they too can be part of evolutionary design. That is not very intuitive. We have found that even researchers well aware of these facts sometimes reason as if whatever has evolved must be universal. They are apparently still influenced by the intuitive picture in unguarded moments.

It is even more counter-intuitive that human nature depends on what is outside the organism as well as what is inside. The idea that our evolved nature is hidden inside, ‘in our DNA’ is very intuitive. How could a species reproduce its typical characteristics so reliably in one individual after another unless there were something inside each individual guiding development? But this idea, for all its intuitive appeal, is no better founded than the equally intuitive idea that if a stone had nothing impelling it forward, it would stop.

In reality, the execution of evolutionary design in each generation occurs because the right factors come together inside and outside the organism. The idea that human nature can be revealed by screening out the interfering effects of culture is fundamentally misguided. It is like trying to find out what bees are really likely by screening out the interfering effects of the hive (perhaps workers are ‘biological’ and queens are ‘cultural’?) We were designed by evolution to develop in a culture just as bees were designed to develop in a hive. The idea that culture and nature are opposing forces in human development is intuitive but mistaken.

Our research suggests that the more flexible view of human nature emerging from current biology will be hard for people to assimilate. Explaining that some behaviours are rigidly programmed by genes inherited from our ancestors, whilst other behaviours are learnt and owe nothing to our evolutionary heritage was easy. It merely applied new labels to what people were already inclined to believe. Explaining that many aspects of our behaviour reflect our evolutionary heritage, but that the resulting design is partly in the human environment, and that it generates diversity as well as uniformity will be much harder.

1 Linquist, S. Machery, E. Griffiths P.E. and Stotz, K., ‘Exploring the folkbiological conception of human nature’, Philosophical Transactions of the Royal Society B, 2011, 366 (1563) pp.444-453.

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