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View Full Version : Book summary : Nature via Nurture



Maciamo
02-08-09, 12:43
It has long been a matter for debate amongst scientists, sociologists, psychologists, educators and philosophers which was more important of nature or nurture.

At one extreme, the empiricists thought that we were born with a blank slate and that environmental factors only, such as education, experience or nutrition, shaped a person's intelligence and character. At the other extremity, the nativists claimed that everything was genetically predetermined through innate instincts and capabilities.

Matt Ridley's book Nature via Nurture, republished under the title
The Agile Gene: How Nature Turns on Nurture (http://www.amazon.co.uk/gp/product/006000679X?ie=UTF8&tag=eupedia-21&linkCode=as2&camp=1634&creative=6738&creativeASIN=006000679X), brilliantly explains how this dichotomy is nothing more than an illusion, and that both nature and nurture are essential, and indeed complete each others. Nurture, Ridley explains, cannot take place without the underlying genes that enable it.

For example, language is acquired through education, but with a malfunctioning or damaged FOXP2 gene people just cannot learn to speak.

The CREB gene allows memory to be formed. Without it we just couldn't form memories. Other genetic variants influence how good we are at memorising things in one way or another. But the knowledge we accumulate though life depends of course of what is available to us in our environment. Genes enable us, give us the potential to do stuff. Our personal experience, the culture we grow in, and and nurture we receive from our parents and teacher make the difference.

Behaviour often require both nature and nurture. People with the low activity variant of the MAOA gene on the X chromosome are more likely to become violent and antisocial if maltreated as children, but slightly less antisocial than average if not maltreated. People with the high activity version are almost immune to the long-term effect of maltreatment. In other words, to become antisocial, a bad environment (here, maltreatment) does not suffice, one also needs a specific genetic variant (here, the low-activity MAOA gene), and vice-versa.

In some cases, genes do act in quite deterministic way. Some (longer ?) versions of the oxytocin receptor genes allow pair-bonding and make animals monogamous (like prairie voles or most birds), while other (shorter ?) versions just render that psychological process impossible and cause animals to lead a promiscuous life (like mice or chimpanzees).

But in most cases genes only contribute to one part of who we are, while the environment does the rest. Fat parents have more chance of having fat children because some genetic variants facilitate weight gain. Tall parents tend to have tall children. But a child with tall parents born in a deprived environment with little food available won't be able to reach his full potential height. This is the developmental bias.

The effect of genes is sometimes counter-intuitive. IQ has been proven to be highly hereditary (as well as highly dependent on socio-economic status). Yet most people would think that education progressively diminish the role of genes on intelligence. The opposite is true. The influence of genes actually increases with age.



The older you grow, the less your family background predicts your IQ and the better your genes predict it. An orphan of brilliant parents adopted into a family of dullards might do poorly at school but by middle age could end up a brilliant professor of quantum mechanics. An orphan of dullard parents, reared in a family of Nobel Prize-winners, might do well at school but by middle age may be working in a job that requires little reading or little deep thought.
Numerically, the contribution of "shared environment" to variation in IQ in a western society is roughly 40 percent in people younger than 20. It then falls rapidly to zero in older age groups. Conversely, the contribution of genes to explaining variation in IQ rises from 20 percent in infancy to 40 percent in childhood to 60 percent in adults and maybe 80 percent in people past middle age.

Schizophrenia

In chapter 4 (pp. 98-124), Matt Ridley explains how a mental condition like schizophrenia can have a variety of cumulative causes. Schizophrenia can be blamed on excessive levels of dopamine in the brain. Dopamine levels vary according to environmental stimuli (e.g. excitement) and can also be altered by food and medication. But the length of dopamine receptors, which determine how much dopamine is absorbed in specific parts of the brain, is genetically determined. Schizophrenia is therefore determined by the type of dopamine receptor genetically inherited and the levels of neurotransmitters determined by nutrition and external stimuli.

Another possible cause of schizophrenia is neither caused by nature nor nurture, but by a virus (environmental factor). If a foetus is contaminated by a herv (human endogenous retrovirus), such as influenza, between the 4th and 6th month of gestation, the proper development of the brain might be compromised, leading to schizophrenia in adulthood.

A fourth cause is developmental, combining both genes and environment. The reelin gene (http://en.wikipedia.org/wiki/Reelin) is one of the organizers of neuronal migration during foetal development. Reduced reelin production has been implicated in schizophrenia, bipolar depression and autism. The cause of reduced reelin is multiple : prenatal infection with influenza virus, epigenetic hypermethylation of DNA, or a mutation in the RELN gene on chromosome 7, among others. Both genes and the environment can play a role.

Finally, schizophrenics have been shown to be deficient in arachidonic acid (AA). AA are apparently leaks too easily from cell membranes in schizophrenics. One way to make the membranes more flexible is to eat more essential fatty acids (EFA's) and less saturated fats, in order to make brain cell membranes more flexible. This change of diet has produced modest improvement in the symptoms of schizophrenics. The cause of AA deficiency is genetic (a faulty enzyme), but it would be exacerbated by environmental factors such as stress, hypoxia (lack of oxygen) during birth, or starvation during pregnancy.

What are genes ?

In chapter 9 (pp. 231-248), Matt Ridley looked at the different ways of defining the word "gene". He found that genes can have 7 possible functions :

1) they are units of heredity that archive wisdom accumulated from millions of years of evolution (Mendel's definition)
2) they are recipes for building proteins through RNA (Watson and Crick's definition)
3) they are developmental switches expressing themselves in specific tissues (Jacob and Monod's definition)
4) they are pangens that can be reused in many different ways inside the body (De Vries' definition)
5) they ensure the healthy development of organs in an expected environment (Garrod's definition)
6) they are selfish replicators striving for their own survival (Dawkin's definition)
7) they are devices for extracting information from the environment (Tooby and Cosmides' definition)