Under the Microscope: The debate over whether our non-physical characteristics are determined by the environment or by our genes, the "nature versus nurture" debate, has a long history, writes Prof William Reville.
At different times, general consensus on this matter has resided at one or other pole of the debate. The current position, informed by much scientific evidence, is that we are shaped by both nature and nurture.
John Locke (1632-1704), the British philosopher, was an early advocate of the influence of nurture. In his book Essay Concerning Human Understanding, he declared that the human mind starts out as a blank slate (from the latin tabula rasa) and acquires knowledge and characteristics entirely through interactions with the world. It follows that everyone is born equal, a concept that impressed Thomas Jefferson (1743-1826), who expressed this notion in the American Declaration of Independence (1776).
The influence of heredity in determining human behaviour and ability received a boost when Darwin and Wallace announced the theory of evolution through natural selection in 1858. Herbert Spencer (1820-1903) coined the phrase "survival of the fittest" to describe how evolution operates, and Darwin's cousin Francis Galton (1822-1911) began the eugenics movement. The term eugenics comes from the Greek eugenes meaning "good birth".
The eugenics movement aimed to improve the genetic composition of human stock by selective breeding - high achievers were encouraged to breed prolifically.
Eugenics was widely popular and was thought of as a most progressive idea. However, the movement quickly went astray. The other side of the coin to encouraging the propagation of superior traits was suppression of the propagation of inferior traits. By 1913, 16 American states had laws allowing compulsory sterilisation of the "feeble-minded". Scandinavian countries introduced similar laws.
The Nazis were avid supporters of eugenics and when they came to power in 1933 they quickly sterilised thousands of patients in mental institutions. The concepts of eugenics and racial hygiene provided "scientific" underpinnings for the Nazi concept of racial purity and the "superiority" of the Aryan race. These awful ideas culminated in the slaughter of millions of innocent "inferiors" in the German death camps.
The unfortunate history of eugenics was followed by an understandable reaction against nature in the nature versus nurture debate, and the pendulum swung back towards nurture. In 1924 the American psychologist John Watson had proposed a theory of "behaviourism" that explains human abilities as the result of interactions with the environment and not at all as influenced by in-built traits. Another American psychologist, BF Skinner (1904-1990), proposed that behaviour is shaped by its consequences, depending on whether they are rewarding or punishing. In his book Beyond Freedom and Dignity (1971), he argued than an ideal human society can only be attained by conditioning people to fit society, and not by society adapting to the needs of individuals.
The extreme position of the behaviourists, ruling out any influence on the shaping of behaviour by factors other than the environment, was shown to be false by the American psychologist Harvey Harlow in the 1950s. Harlow took baby monkeys away from their mothers and reared them in cages that had two artificial mothers. One was a simple wire-frame mother fitted with a feeding bottle of milk. The other was a more life-like cuddly model, but it had no milk. Behaviourism predicted that the baby infants would spend most time with the rewarding wire-frame mother. However, the baby monkeys spent most time with the cuddly mother, making brief visits to the wire-frame mother only when prompted by hunger. Their mother model preference was being prompted by an inbuilt expectation that a mother should offer warmth and love.
A wealth of experimental evidence has since shown the importance of heritability in the determination of many human abilities and characteristics. Studies of identical twins reared apart have been particularly important. Each twin is genetically identical to the other, so differences noted between them as adults must be ascribed to environmental influence.
Such studies have shown, for example, that about half of the variability in IQ between people is accounted for by genetic heritability. The situation is similar for success in life and personality traits.
It is now clear that the way we turn out as human beings is the result of interactions between our in-built inherited potential, the environment in which we reside, and a dollop of pot-luck. Over the years, eminent people, emotionally convinced that either heritability or environment were overwhelmingly important, have swayed the majority to their point of view. And there is still a tendency in certain influential quarters to ascribe undue importance to the shaping effect of environment. This, I believe, is largely caused by egalitarian sentiments. Although it is well-meant, it is unhelpful simply because it is wrong.
One consequence of these notions is to put a huge unnecessary burden on scrupulous parents. Believing that the environment is everything, such parents will make huge efforts to raise their children in the perfect manner. And then, as can easily happen, little Johnny or Mary goes wrong. The parents are now wracked with guilt, feeling that, somehow, somewhere along the line they provided an inadequate environment.
If you find yourself in that situation, you should stop beating yourself up. Parenting skills are important, but how a child turns out depends on much more than this. If you have done your best, that has been, and will continue to be, a force for the good. It is an important factor, but only one, in a tapestry of interacting factors.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie