1 Individual differences

1.1 Introduction

This course addresses the question of how the differences between individuals, especially in behaviour, arise during development. Development, the transformation of the single cell, the zygote, into an adult organism with billions of cells, numerous organs and an intricate, functioning nervous system, is one of the most remarkable feats of living systems. The process begins when an egg cell, or ovum, is fertilised by a sperm, or spermatozoon. The resultant single cell, the zygote, divides to produce two cells and each of these divides again to produce four cells; and division continues apace producing the thousands and eventually the billions of cells of the adult organism. The cells grow, interact, move around and diversify under the guidance of the genetic material each cell contains, producing different structures and features and the characteristic shape of the organism. The sequence of events that occurs during that period of rapid change that begins at conception and continues through to the time when the relative stability of maturity is attained, i.e. development, has been described in considerable detail for many organisms. This course focuses on individual differences and, in particular, on how those individual differences arise.

There are two key causes of individual differences between organisms. Firstly differences in genes between people can manifest in differences in their nervous systems and thereby contribute towards differences in their behaviour. The way in which genes affect behaviour, or the way differences in genes translate into differences in behaviour, is a recurring theme throughout this course, but is considered explicitly in Sections 1.2 and 8.

Secondly, the developing body acting in dynamic interaction with the environment. There are three consequences of this dynamic interaction, which lead to three different kinds of question, and they in their turn lead to three different kinds of investigation. The dynamic interaction is continuous and the environment constantly changing but the situation can be simplified by considering a single, identifiable environmental event. The three questions then become:

1. Is the behaviour of the organism affected at the time of the event?

2. Is the behaviour of the organism affected at some later time, after the event?

3. Through what mechanism is the interaction, between the body and the event, mediated?

These questions can be represented on a schematic diagram of development (Figure 1).

Figure 1 Schematic representation of five stages of development. The length of each stage represents the amount of change that occurs during that stage, not the duration of each stage. The circled numbers represent questions 1, 2 and 3 in the text

So, for example, if the event (A in Figure 1) were high levels of physical contact during infancy (i.e. lots of cuddling), then this might affect (a) the behaviour of the infant at the time (effect 1 in Figure 1) and (b) the behaviour of the child/adolescent/adult years later (effects 2a, 2b, 2c respectively in Figure 1). If there is an effect, then there is a mechanism by which physical contact caused those effects (3 in Figure 1), and this mechanism can be sought. Note that in Figure 1, the length given to each stage of development represents the amount of change that occurs during that stage, not the duration of each stage.

The first part of this course (Sections 3 and 4) considers questions 1 and 2, which can loosely be regarded as the phenomenology that is the conspicuous consequences of developmental events. The second part of the course (Sections 5, 6 and 7) considers question 3, the mechanisms by which events during development exert their effects. The final part of this course, Sections 8 and 9, returns to the issue of genes and their influence on development, the nervous system and behaviour. To finish this introduction, the problem of relating genes to behaviour is considered more fully.