Bruce Hood - The Domesticated Brain - A Pelican Introduction (Pelican Books)

Identical twins occur when a fertilized egg splits in two soon after conception. In the rare cases of conjoined twins, that separation is incomplete. Identical twins share all their genes whereas non-identical twins, who come from two separate fertilized eggs, share only half of their genes. Like Tweedledum and Tweedledee from Alice in Wonderland , identical twins look the same, behave the same and often think the same thoughts. There is even a popular myth that twins are telepathically connected and read each other’s minds.

Studying twins is important for working out the roles of genes and environment in shaping the course of development. Like Daisy and Violet, twins are sometimes adopted, but unlike conjoined twins, they can be fostered out to different households. By comparing twins, identical and non-identical, raised in the same or different households, you can estimate how similar they are and then work out the relative contribution of genes and the relative contribution of the environment.

These adoption studies show that identical twins raised separately are more similar than non-identical twins raised by different families. This proves that aspects of personality and intelligence must be heritable. But identical twins are not identical. Even as conjoined twins, Daisy and Violet had marked differences in personality and allegedly even different sexual orientations, but they were hardly the same person. 38When it comes to personality and intelligence, heritability only accounts for, at best, half of the overall similarity. This is an important point that Judith Rich Harris draws our attention to in her book No Two Alike 39. We are so used to thinking of identical twins as being identical, we fail to realize how different they actually can be. If you think about it, Daisy and Violet Hilton not only shared the same genes but they literally shared the same environment. How could they be so different?

Most people believe that one of the main reasons that individuals can be so different is because they were raised in different homes. The history of parenting is full of advice about how best to raise children and the bookstores have whole sections dedicated to parenting manuals. This comes from an understandable concern to look after our offspring and give them the best start in life as well as deep-seated beliefs about the forces that shape individual development. We have all grown up in a variety of households with different experiences that have shaped us, which is why there is a common assumption that we are what we are because of the way we were raised. When we blame delinquent children, we typically look to the parents. However, Harris spent many years surveying the fields of developmental psychology and concluded that when it comes to psychological outcomes such as intelligence and personality, neither genes nor the household environment can predict how we will turn out.

Ironically, that is a message that most parents probably do not want to hear, but they should be the first to agree with Harris. Any parent should be able to confirm that no matter how much they try to treat their various children equally, they end up very different. In fact, when the proper measurements are done, two siblings raised in the same household are not much more similar than two randomly selected individuals of roughly the same age plucked from the same population. Despite what most parents want to believe and parenting manuals promote, the home environment plays a relatively minor role in shaping the development of children.

If it isn’t the home environment and it cannot all be the genes, then what explains individuality? Harris argues that the major determinant of a child’s intellect and personality is the influence of their peer group – other children. While the child may behave according to their parents’ expectations in the home, they put on a different face in the playground and shopping mall. Children act and respond to others differently in different situations. This is why children of immigrants do not learn their parents’ accents when learning English, but adopt the local dialects and accents of the neighbourhood kids.

Harris’s thesis is highly controversial as it goes against the modern trend for parenting expertise. It is also leaves out the extreme environments of Romanian orphanages and depressed mothers who have been shown to affect long-term development. Moreover, parents indirectly influence which peer groups children are exposed to because they choose the neighbourhoods and schools that their children end up in. That said, the goalposts are likely to shift again when one considers the pervasive role that social networking sites such as Facebook and Twitter now play in teenagers’ lives. However, even if today’s extensive networks outside the home play a greater role in shaping children, this cannot explain why Daisy and Violet, who shared the same genes, the same environment and the same peers, were still different. Perhaps it’s because people treat identical twins, even those conjoined at the waist, differently so as to distinguish them. That seems plausible, but a more likely explanation is in itself unlikely – and that is the role of random events in development: an area of research known as epigenetics .

Epigenetics

What do the sex of a clownfish and the spread of the common cold have in common? A strange question maybe, but both are examples of epigenetic phenomena that are triggered by social behaviour. They both depend on the interaction of biology and the influence of others. Epigenetics is the study of the mechanisms of interaction between the environment and genes – the way that nature and nurture work together.

Epigenetics provides answers to the sorts of common questions we all ask ourselves. Are we born mad, bad or sad, or is our personality determined by events in our lives? Why are our children so different when we try to treat them equally? These questions are at the heart of how best to create the societies we wish to live in; often shaped and controlled by government policies and laws. The answers people prefer to give to these questions come from deep personal opinions and reflect their political persuasion about the role of the individual in society. However, epigenetics offers a new perspective to understand human development that combines our biology with our experiences.

As we noted earlier, genes are the strings of DNA molecules, found in every living cell, that instruct the cell what to become. They do this by building proteins from amino acids, which in turn are made from combinations of atoms of carbon, hydrogen, oxygen and nitrogen. Every cell in the body has thousands of proteins and DNA determines what type a cell is and how it operates by regulating the production of proteins. Genes are like books in a library that contain information that needs to be read or transcribed in order to build the proteins. The proteins instruct the cell to become something, such as hair follicles, while others can turn them into neurons. This is a very simplistic account and there is considerably more to the story of the mechanism of genes, but for the level of discussion here, it is sufficient to know that genes are like sequences of computer code within the cell that control its operation.

Genes build humans and humans are very complex animals. Each body is made up of trillions of cells and the initial speculation was that humans must have a considerable number of genes to code for all the different arrangements of cells in our bodies. In 1990, scientists working on the human genome project began to map the entire sequence of genes for our species, using sophisticated technology that enabled computers to read off the sequences as strings of code. Very soon, it appeared that initial estimates of over 100,000 genes had been way off. Although the project is still continuing, at the last count it would appear that humans have only 20,500 different genes. That may still sound like quite a few but when you consider that the humble fruit fly, drosophila , has 15,000 genes, humans look decidedly puny in the genetic endowment department. In fact, much simpler living things like the banana or the rather revolting roundworm have more genes than humans and, as if that were not enough, the organisms that have the highest and lowest numbers of genes are both sexually transmitted diseases, trichomonas vaginalis with 60,000 and mycoplasma genitalium with 517.