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History of the biological difference

Different epochs have expressed different scientific views of the differences between men and women, elaborated on the basis of the knowledge of the time but also inspired by the prevailing concepts of the family and of social structures. They were in addition usually marked by a strong “male-centred” ideological stamp, evolved in order to give the idea of female inferiority a scientific basis.

In the 19th century anthropologists used to study human beings, analysing and measuring forms and organs of the body: they thus declared that women’s brains were more like those of anthropomorphic monkeys than those of the male of the human species, thereby successfully showing that women have weaker intellectual capacities. Physiology also followed this route: Carl Vogt, a German, held that women more often than men presented animal-like features, such as a prominent jaw. Moreover he shared with many other scientists the opinion that on the evolutionary scale women occupied a place half-way between monkeys and men.

scene from the film “2001: Space Odyssey” by Stanley Kubrick (1968)

In the last decades of the century Cesare Lombroso developed his theory on the atavism of certain forms of human behaviour and declared among other things that women, animals and children show a higher tolerance of pain. He did not consider this characteristic as a positive fact but rather as a trait close to bestiality. In the same period Darwin argued instead for male superiority in evolutionary terms, making the male, more active, inventive and inclined to changes, the motor of evolution. Based on this hypothesis science for many decades science imposed research into the differences – and not only cognitive – between men and women.

In the early 19th century the metabolic theory of sex, formulated by Patrick Geddes and John Arthur Thomson, became dominant. They claimed to found this same male/female-change/preservation dichotomy only on the basis of experimental biology and on the mechanistic laws of physiology. Geddes and Thomson thought that development in the female or the male depended on the metabolic condition: they supposed that women conserved the energy produced by cellular processes while males used any surplus energy. At the moment of the determination of the sex of the embryo, its development in one direction or the other depended on the type of metabolism operating, also in relation to environmental conditions: in conditions of abundant nutrition the female phenotype got the upper hand while in adverse conditions it was the male. In the vision of these two scientists, the metabolic condition also exerted an influence on psychological and behaviourial aspects, women being passive, conservative and not particularly interested in social issues and politics, and males enterprising and active, oriented to the public sphere.

A new model, the hormonal model, which gained ground in the 1920s, nourished a large current of research which nevertheless failed to produce unequivocal results. The steroidal sex hormones are secreted from the testicles and from the ovaries, which represent the most important sexual dimorphism, but they are present in both sexes, albeit with different profiles of expression. And the action of the sex hormones is of a specific type, masculinizing or feminizing, independently of the sex of the individual in which they are active.

The discovery of the different structure of x and y chromosomes in the two sexes also dates back to the early 19th century. However, it was to attract the greatest attention only in the 1950s, with the study of chromosomal anomalies. In the ensuing decades the research into sexual differences in the x and y chromosomes led to attributing to males with xyy the supermale syndrome, based on the presupposition that an x with a double y chromosome doubled the subject’s masculinity and, directly influencing behavior, resulted in a more active and aggressive sexuality. After a few years of authoritative publications on the subject, a meticulous epidemiological study demonstrated the groundlessness of the attribution of genetically violent behavior to males with xyy.

The fundamental turning point came in 2001: the entire sequence of the human genome was published and since then research has been oriented to the identification of measurable differences in the male and female genomes which can shed light on the influence of sex and gender on physiology (including the cognitive differences), physiopathology and human pathology. It is obvious that an increase of knowledge in this field would have profound effects not only in the medical field.

Since then research has focused on the disciplines of genetics, genomics, and epigenetics (the study of inheritable modifications which alter genetic expression, but not the sequence of the dna). Research into the sexual differences in the human genome, known as “sexoma”, proposes to describe in the language of genetics the already recognized differences, but also to discover others that are still unknown.

The importance of the new field of studies has given rise to specific institutions: the Society for Women’s Health Research, founded in 1989 by an association of American obstetricians and gynaecologists, chose as its mission not only to be concerned with women’s health in relation to reproduction but also to broaden the objectives of its own research into gender differences in the occurrence and treatment of pathological conditions in general. This work produced as its first result the report Exploring the contribution to human health: does sex matter?, published in 2001: this report affirms that sex has a profound effect on the function of cells and organs, and that it is necessary to understand its influence on biology, from the molecular level to that of the whole system, including interaction with the environment.

In 2005 the journal Nature published an article which reported on the sequencing of the human chromosome x and the difference in the two genomes, male and female, was very much higher than was believed until then, even higher than the difference between the human genome and that of the chimpanzee. Subsequently this differencewas strongly down-graded, but above all it was understood that the study of the influence of genes in determining the sexual characteristics is very complex because it concerns not only the scaled-down structure of the chromosomes and genes but also the molecular and epigenetic mechanisms which control genetic expression. The action of the genes must also be contextualized in a physiological system in which to observe the effects: the same gene, equally expressed in males and females, might have different effects.

James Tissot, caricature of Charles Darwin for the magazine “Vanity Fair” (1871)

In the light of the new discoveries there has thus been a return to studies of the brain: a controversy still under way is focusing on the possible dimorphism between the male and female brains. Hence it is also focusing on the possibility that a dualism of the cerebral structures might exist which is linked to sex, on which a sex-specific psychological configuration could be established. One example is the research on the corpus callosum, a cerebral structure which has long been studied as an element able to explain the differences between males and females in behaviour and in cognitive capacities.

In an article published in 2015, Sex beyond the genitalia: The human brain mosaic, the authors have tackled the question of the functioning of the brain in the two sexes, quantifying the structural differences in a broad sample of people through magnetic resonance data. The results obtained show that human brains do not belong to two classes, “male” and “female”, but that the differences connected with sex can be described as a mosaic of characteristics, some more frequent in women, others more widespread in men, and yet others equally expressed. Once again, there is no certain and unequivocal answer.

The biological vision of sexual differences has thus changed over time, following the prevailing theories and ideologies in every historical context, as well as the concerns of medical research. To identify and define the essence of the difference between the sexes the reproductive organs have been called into question, the colour and density of the blood, the size and the lateralization of the brain and hormones. Today in “advanced” societies in which the male/female concept is fluid and controversial, the language of genomics is the one preferred to explain and describe the differences of sex and gender. But the question remains of how far these differences, different from those connected with reproduction or under the influence of sex hormones, are rooted in biology and what their biological determinants actually are.

The power of this question lies in the authority recognized in the scientific method of providing objective answers: differences that might be rooted in the biological constitution of the sexes would be universal and natural for these categories. Biological determinism has very serious implications: it might justify, for example, recourse to categories such as sexual hierarchies, and it is for this reason that great caution is required in evaluating scientific data proposed as a basis and a monocausal explanation of the differences between men and women.

Mariella Balduzzi

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