Nature or Nurture?

A question that frequently arises is whether our brains, and therefore our personalities, are fixed from birth, or can they change over time?

The answer is that both occur. We will begin by considering the way in which our brains develop; from the first brain cell (or neuron) to the end of our lives. It all begins in the womb when, on your 42nd day of life, the first neuron develops. During the next 120 days your brain will develop around 86 billion neurons at an average rate of more than 8,000 per second!

About 60 days before you are born, each neuron begins to establish connections with other cells in the body. In some cases these connections are electrical and in other cases chemical.

The number of connections each neuron will establish will vary from a few thousand up to 100,000. By the time you are three, you will an average of 10,000 connections (known as “synaptic connections”) for each neuron. To put that into perspective, the Amazon Rain Forest consists of roughly 2.4 million square miles of forest. So if we suppose that in that area there would be around 86 billion trees, and that each tree had 10,000 leaves, your brain would be like all the leaves in the Amazon Rain Forest packed inside your head, making it by far the most complex organ in your body.

However, unlike any other organ in your body, after the age of 3 your brain actually starts to get smaller.

As you move from being a baby to becoming a toddler, the process of "synaptic pruning" begins. This is a process by which the synaptic connections you use on a regular basis become stronger, and those that you do not use get weaker, or even disappear altogether.

As a result your personality becomes imprinted on your physical brain – psychologists sometimes refer to this as the "imprint period".

By the time you are 16 you will have lost around half of your synaptic connections and the basic foundation of your personality, your character and your values will have been established - you are who you are!

However, although our personalities may be relatively well established by the age of 16, the brain has the ability to continually reorganise the neural pathways in the light of experience. This is referred to as "brain plasticity" or "neural plasticity". As we learn, we also develop new synaptic connections.

So although the overall number of synaptic connections tends to decline throughout our lives, the nature of the connections within our brains is constantly changing.

One of the pieces of break-through thinking in brain research was when scientists realised that the brain was like a series of muscles that could be developed and strengthened through use. As evidence of this, a study of the brains of London taxi drivers in 2000¹ found that they had larger posterior hippocampi than most people, which the researchers attributed to the extensive amount of navigational memory required for their work.

¹ Maguire, E.A., Gadian, D.G., Johnsrude, I.S., Good, C.D., Ashburner, J., Frackowiak, R.S., Frith, C.D., 2000. Navigation-related structural change in the hippocampi of taxi drivers. Proceedings of the National Academy of Sciences of the United States of America 97 (8), 4398–4403.

Additional reading:

How can Neurological Dominance be measured?