What makes one child grow up to have one set of personality characteristics, while his brother or sister grows up to be a very different person? The answer that seems obvious to most people is that the two individuals have different experiences, but it's been difficult to link particular experiences with particular personalities. Even the early events like weaning and toilet-training-- once thought to be so significant to personality-- do not seem to be connected in any clear way to personality variations.
What does seem clear is that, whatever effects experience has, there are some biological factors that help to determine personality. These factors are present from birth and help explain why even very young babies respond differently to the world.
Biological factors determine the individual's "temperament", the group of personality characteristics that seem to be present in some form from early life onward and that make us consider people as having individualized personalities. Temperamental characteristics are expressed in different ways as the person matures, but are always there, no matter whether experiences tend to encourage or discourage them. One example of a temperamental factor is activity level. This does not refer to so-called "hyperactivity" in a pathological sense, but simply to the preferred amount of activity with which an individual feels comfortable. This can vary a good deal within a normal range, with both quite active and quite inactive people as examples of normal variations. Another is mood quality, with normal differences of cheerful or morose dispositions appearing not just when good or bad things happen, but in neutral situations.
Activity level is a good example of how a temperamental characteristic is expressed in different ways as the person matures. A newborn baby of active temperament cannot jump up and run around, but can wriggle, kick, wave its arms, and shift facial expressions. Two years later, the same person covers a lot of space and does a lot of things, and is still evaluated as active because he or she does more than most other children of the same age. By age 10, and certainly by adulthood, the person shows less activity than at age 2, but is still a more active person by choice than most others of the same ages. Activity level is also a good example of how a child's temperament may or may not be a "good fit" with his or her environment. Sports-loving parents living in a roomy house with a big yard will probably respond quite differently to their active child than will a quiet, scholarly pair of apartment-dwellers who treasure their collection of antique porcelain.
How do biological factors determine temperament or any other aspect of personality? The obvious answer involves genetic material and its influence on the nervous system, and this idea has been very popular since research established a connection between depression and the neurotransmitter serotonin, and between serotonin levels and a specific gene variant. It was thought that the gene variant helped determine whether a person would respond to life stresses by becoming depressed.
More recently, though, further questions have been asked about the gene-environment-depression connection (Constance Holden, "Back to the drawing board for psychiatric genetics", Science, 26 June 2009, Vol. 324, p. 1628). Analysis of 14 studies indicated that going through stressful events was likely to cause depression for many people-- but showed no evidence that any specific gene variant was connected with greater or lesser vulnerability.
Researchers investigating this issue are speculating that the connection between genetic make-up and depression are probably much smaller than was thought. But they also point out that it is likely that dozens of genes, rather than just one, would be responsible for neurotransmitters like serotonin. Choosing just one to look at is probably a mistake, because groups of genes work in cooperation and therefore the whole genome needs to be studied.
It is certainly not clear to what extent the tendency to depression, or any temperamental characteristic, is inherited or predictable from parent characteristics. However, as there seem to be many genes involved, it is not likely that there would be a simple dominant-recessive inheritance pattern, like the one that occurs with human eye colors. Nor is it clear whether both genes and environmental events (even prenatal occurrences) might need to cooperate in order to produce certain temperaments. The only thing generally agreed on is that most personality characteristics are not caused by simple experiences working alone.