In my last article, I addressed the question of how much individuality is rooted in genetic differences. From older evidence (twin studies) and new genetic evidence (genetic differences tightly correlated with five personality traits), one concludes that genetic differences can influence one’s personality. This had been a long-standing question from well before there was a proper science of genetics but only an idea of human heredity. It is good to now have some scientific evidence relevant to it.1
Yet, leaving things there might give the impression that genes are the whole story of what underlies our individuality. That is untrue. This article is a follow-up and an attempt to indicate the complexity of the situation. Here, I will give a brief explanation of how one might think about this matter in terms of four general factors that contribute to human individuality. Each such factor is actually a class of elements, each quite heterogeneous, and that heterogeneity contributes to the range of outcomes, that is differences amongst people.
Perhaps little here will be completely new to you but the “developmentalist perspective” I give might be useful in your thinking about this matter. How and why humans differ from one another is, after all, a fundamental and interesting issue. Despite all the similarities we humans share, there have never been two individuals wholly alike, not even identical twins, amongst the 7.5 billion human beings alive today or the billions who have preceded us and passed on.
Here, I first list the four classes of factor that shape our differences. As mentioned in the previous article, the early idea was the matter could be reduced to two elements, namely “nature” and “nurture”. Today, people more often refer to genetics (instead of “nature”) and environment (instead of nurture). In either formulation, the implication is that the two categories were both distinct and, in some basic sense, additive. Neither inference is true.
Using the modern formulation: while “genetics” is a reasonable category, “environment” is not. It is far too heterogeneous. Employed in this context, it basically means “everything else that is not DNA”. It is far more sensible to split “environment” up into three categories: 1) external environmental factors, 2) experiential ones, and 3) internal developmental ones. With genetics as no. 4, these make up the four sets of fundamental factors that contribute to an individual’s development. They are not separate and independent but converge in the phenomenon of biological development, as will be explained. Let us take them one at a time.
External environmental factors. This is the first and simplest factor of the four. Every animal has certain externally-supplied basic requirements to live: oxygen, adequate food, and a certain range of temperatures to survive. These conditions are necessary for, and permit, healthy development and growth. If either food or oxygen is in too short supply for too long, the animal’s development or health is harmed, with possible long-lasting effect on the character or health of the individual. The same is true if the animal is forced to live outside its normal temperature range for any length of time. Although different animals of the same species exposed to these sub-optimal conditions may experience the harms to different degrees, with resulting differences in their character, these are probably the least interesting source of individual differences.
Experiences. As a shaping factor, this is obvious but still worth discussing. Indeed individual experience was for decades, the focus of psychological work on individuation, particularly in the Freudian school of thought. Even before Freud, however, it had been obvious to thoughtful people, for centuries, probably millennia, that individuals who came from happy families tended to be more sociable, happier, generally more functional, than people who grow up in families where one or both parents had major personality problems or drug addictions such as alcoholism. Experience matters!
What was not understood clearly until the late 20th century was that individuals developing under adverse circumstances often develop subtle biological changes, involving hormonal shifts or changes in the neural circuitry of the central nervous system (CNS). These are easiest to detect, investigate and treat when extreme events have been encountered, whether as a child or an adult. Such post-traumatic stress disorder (PTSD) was first detected in soldiers during WWI and given the label “shell shock”. At the time, it was often dismissed by people in military authority as a failure of character of the soldiers who were experiencing it.
By the last two decades of the 20th century, there was much better understanding of PTSD; it was recognized as a medical problem involving alterations in the nervous and hormonal systems that did not go away when the stressing stimuli ceased. In the early 20th century, a major advance had been the discovery that the mind could influence the body, via so called “psychosomatic events”. By the late 20th and early 21st centuries, there had been complementary advances in understanding how stress in the body affected the mind. Too often, when the disturbance had been great, it became locked in as both a physiological response and an on-going emotional disturbance. This realization is given in the title of a book on such traumas, “the body keeps the score”2.
While these observations illustrated how abnormal experiences can alter previously normal biological functioning, others indicated how normal experience helps create normal biology. Take the example of vision. In experiments that might not be permitted today, it was shown that kittens who had been blindfolded for the first three weeks of their life, and then had the blindfolds removed, were essentially blind the rest of their lives. Their visual nervous system had had its normal development interrupted because the kittens were not allowed to see when that system was developing. Evidently, the normal early experience of seeing is necessary for full and healthy development of vision for the rest of the animal’s life.
Something similar is necessary for the development of normal language ability, both speaking and understanding. This conclusion is based on a handful of incidents of children who had been separated from their parents and raised by other mammals, in particular wolves and several different species of monkeys. They were later taken into human society and socialized. These children, with one possible exception, seemed incapable of developing full human language capacity or complete socialization. They otherwise seemed to have good basic human intelligence3.
The internal “environment”. In normal speech, the word “environment” designates the whole area outside the body which influences it. In effect, “environment” denotes the external environment. If, however, it is taken to indicate everything surrounding the genes – as in the apposition of “genetics vs. environment” – then it includes all the internal biological material and structures in the body that are not DNA.
That internal environment is intensely dynamic and in fact, incredibly diverse throughout the body, differing from tissue to tissue and organ to organ. Of course, since the body functions as a whole, those spatially separate internal environments are linked in their operation. The crucial fact is that all of the dynamism and diversity of changes the individual undergoes, in their growth, development and physiology, are driven by the operations of its own internal processes.
Those characteristics of dynamism and diversity are on most dramatic display in the first nine months of human life, when the initial stage, the fertilized egg, develops through the processes of embryogenesis and foetal development to produce a new-born human baby, usually one kicking and screaming and undoubtedly baffled at its sudden change of external environment (from the warm aqueous surrounds of its mother’s womb to the unprotected space of open air, with large creatures (adult humans) moving around). These processes of internally driven but quite standard changes in the internal environments, of mind-boggling complexity, are given the collective term “development”. Of course, human development does not stop with birth but continues, though at a slower rate, from birth through the teen-age years to early adulthood4.
There is nothing comparable in complexity to biological development in the world of non-living things. Beginning in the 17th century, it became fashionable to liken the living creature to a machine. Yet, as we have learned more and more about living things and their astonishing chemical and structural complexity, we can safely say that no machine is remotely as complicated as living things. And no machine (yet!) can construct replicas of itself or repair damage to itself in the ways that living things can.
Perhaps just as astonishing as is the complexity of biological development is its reliability. For each species, it works remarkably well from the earliest stages to the point at which the individual animal is ready to reproduce itself. It is not perfect of course – there are some deaths due to errors in development at every point in the process – but it is tremendously steady and reliable5.
Nevertheless, there is enough flexibility, combined with the resilience of the process, to allow slight individual differences in the process to occur, which then translate into the physical differences we can see amongst people. Such minor deviations in the development of the neural system can also lead to the personality differences that develop. What causes such differences? It is here that we return to the matter of genes and genetic differences.
Genetics: the role of genes in development. Genes provide crucial input at every stage of development. Unlike the outwardly visible events of development, which can be followed with sophisticated optical (and other) methods, the world of genes and gene action is considerably harder to track. Indeed, in humans, it is even harder than it would be, for instance, in mice, whom we are allowed to experiment on and “sacrifice” (i.e. kill). Obviously, that latter freedom does not apply to humans nor, of course, should it. (What we do to other animals by way of experimentation to gain knowledge is a subject of on-going ethical debate.)
Yet, while the details of how genes are involved in the development of an animal are immensely complex (and of course different in different animals), we can make a useful generalization. What happens at each time point in development is the expression of a set of genes that interact with each other in set ways. We call such sets of interacting genes “gene regulatory networks” or GRNs.
Picture a flow chart of an industrial organization with some high-ranking figures at the top who give orders to a second-tier of managers who then do the same either to a third tier or to the workers who actually produce the product(s) of the factory. But now imagine that the top group of managers disappear and some members of the lower tiers either become major managers who now start whole new directions of the manufacturing process OR where the products morph into new managers and do the same! (This is a nice illustration of the limits of similes and metaphors when applied to biological processes.)
A key additional point is that small differences in the players at each point can make for large differences in how the process plays out. The consequence is that the ultimate products of the process can differ, in their amounts and activities, depending upon the differences in the managers or the final workers! This would be a terrible weakness in a manufacturing process but in biology it helps create the wonderful diversity of individuals in each species.
Thus, instead of a static flow-chart representation, the developmental process for something as rich in detail as the development of an animal from a fertilized egg to a new-born can be better represented as a film of ever-changing flow-charts of a theatrical production. In this situation, the major actors (the genes or rather the products of those genes) are continually being hustled off the stage to be replaced by others, while the over-all size of the cast on stage is expanding and splitting into groups doing their own thing (in the case of the embryo, this would be building a new tissue or organ or limb). This may seem both confusing and hard to visualize but that is biological development, an immensely complicated subject.
Summing up: We began with a fairly simple question and a fairly simple proposition. The question was: what are the factors that make individuals different from each other? The proposed answer was that there are four classes of factor that contribute to the formation of those individual differences. I then discussed each of those categories and tried to explain how each one can contribute to differences in individuals. In our collective understanding of human individuality, we are moving from the not very helpful “nature vs. nurture” dichotomy to a subtler and deeper grasp of what is involved. In this series, we will soon return to matters of biological development and how they affect human beings.
Supplementary reading
Ball, P. (2023). How Life Works: a user’s guide to the new biology. Picador:
London
This is a very thorough and clear recent account of how to think about biological development and its multifarious aspects.
Martinez-Arias, A. (2023). The Master Builder: how the new science of the cell is rewriting the story of life.
Another highly useful general treatment of development, focused on the biology of cells, which are the living “building blocks” of development in animals and plants.
Wilkins, A.S. (2007). Between design and bricolage: genetic networks, levels of selection and adaptive evolution. Proc. Natl. Acad Sci USA, 104, Supp. 1, 8590-8596.
A relatively short account of GRN’s and their importance in development (and evolution).
Two days after posting the previous article, I came across one in The Guardian about six pairs of identical twins separated at birth, raised in different families, who only much later discovered each other and their great similarities in personality. Such similarities are most readily explained on the basis of their essential genetic identity. See Isabelle Aron (2024). Seeing Double, The Weekly Guardian, 4th October, 2024, pp. 40-44.
See The Body Keeps the Score: mind, brain and body in the transformation of trauma by B. van der Kolk (2014), Penguin Books
For a wonderful film portrayal of this, based on a true story, see François Truffaut’s The Wild Child (1970). The Wikipedia entry on “Feral children” is good.
Of course, bodily physical changes do not cease at the end of the teenage years but continue in the form of senescence (see no. 15 in this series, “Ageing”). Senescence, however, is best considered as deterioration rather than development.
It is estimated that about 30% of normal human conceptuses abort at fairly early stages due to errors in development. The American “right-to-life” anti-abortion movement has nothing to say about such natural abortions.