As was well described by Zazzo (1960), twins have a specific psychic development, which he named a “screen effect”. Each twin acts as a sort of “screen” between the world and the other twin: mutual inter-identification, leading to a partial fusion between the self and objectal representations, which causes the two individuals to merge. This can cause delays in individuation and body image disorders such as recognition in the mirror at about three to four years of age instead of two and a half.
Zazzo thus considered twins as a “couple”, not as doubles, which explains the differences between twins, especially monozygotic ones. The personality is formed and transformed in and by the couple, each creating its roles according to those of the other, resulting in two complementary personalities. Thus, we see personality differences such as dominant/dominated and extravert/introvert.
Environment also causes differences: the parents of twins tend to appreciate the character of one always compared to the other.
With respect to friendship, monozygotes share 50% of their friends, same-sex dizygotes 25% and opposite-sex dizygotes 5%.
The twin relationship becomes stronger around the age of 36 months, once the separation–individuation process is completed. During adolescence, the traditional revolt against the parents may also be directed towards the other twin, which shows ambivalence between the desire for differentiation and that of twinning, which will produce an increased frequency of psychological disorders in monozygotic twins with psychosomatic and depressive disorders in the dominated twin and behavioral disorders in the dominant twin.
That said, ambivalence is not specific to the twin relationship, it can be observed in all sibling relationships: in the same way as “ordinary” siblings, twins develop relationships of closeness and conflict, of love and hatred, two complementary aspects called for in the definitions of sibling relationships. Proximity and conflict are present in twins in the same way as in ordinary siblings; ordinary and twin sibling relationships seem to be built around these two central, common and complementary dimensions, “warm” and “conflictual” relationships.
Three characteristics inherent to twin pairs may influence twin relationships: type of twinhood, gender and temperament of the children:
– monozygotes have higher scores on positive aspects of the relationship, such as companionship and empathy; dizygotes, on the other hand, have higher scores on aggression, rivalry and avoidance, although this difference has been observed in adolescence, not in infants;
– girls score higher than boys on empathy, companionship and guardianship, while boys score higher on avoidance;
– emotivity negatively impacts twin relationships in adolescents averaging 12 years of age, while sociability and activity have been associated with positive aspects of twin relationships.
Finally, there are differences in the perceptions of twin relationships between children and their parents. Indeed, the more children emphasize warm and conflictual relationships, the less their parents perceive them. Similarly, mothers value the positive aspects of the relationships, whereas adolescents are more critical.
It could be that a fundamental difference exists between what happens within the twinned pair and what the pair actually shows to outsiders, including their parents.
1 1 As we like to joke, it is often said that the populations studied by university psychology teachers are... their students.
2 2 We know the formula: what does a human baby look like? A chimpanzee fetus.
3 3 We know Freud’s famous saying, “Whatever you do, it will be wrong”.
4 4 “Tanguy” was a very popular French movie 20 years ago, Tanguy being a nearly 30-year-old son showing his desperate parents no intention of leaving the family home...
2
The Fetus and Fetal Life
2.1. Conception and medically assisted procreation: children born through medically assisted procreation
In order for a child to be born, there must first be procreation. Since the beginning of the 1980s, medically assisted procreation (MAP) has become increasingly frequent: in 2018, more than 25,000 births, or 1 child in every 30 (which is still modest compared to the 800,000 annual births in France), were conceived by MAP (IVF, DAI1). This increase in recourse to MAP could be explained in part by an alteration in fertility. Studies point in particular to smoking, obesity, sexually transmitted diseases and endocrine disruptors.
With regard to testicular dysgenesis syndrome (TDS), studies have observed an overall change in male reproductive health, probably since the 1970s, consistent with the sperm quality data observed internationally.
These trends appear to be more consistent with environmental changes (such as increased ubiquitous exposure to endocrine disruptors since the 1950s and increased female smoking) than with genetic variations in the French population.
Following up on the health of these children could constitute a major public health issue since the follow-up of the infants reveals increased perinatal complications (prematurity, hypotrophy and neonatal mortality) and an increased risk of congenital anomalies (cardiovascular, metabolic or urogenital system) compared to naturally conceived children.
However, many confounding factors exist, such as multiple pregnancies. There is an increase in birth weight after frozen embryo transfer compared with fresh transfer, an increase in epigenetic diseases and certain solid tumors among children conceived by MAP, regardless of whether the IVF or ICSI techniques are used. However, the absolute risks remain moderate and the prevalence of these pathologies remains low.
After adjusting for confounding factors, the data is reassuring, with statureweight, cognitive and psychomotor development equivalent to that of naturally conceived children. It would seem that the occurrence of chronic pathologies in adulthood can be attributed to an embryo-fetal origin, hence the need for long-term follow-up of these children, who are still under-studied, as well as the investigation into the causes of these observations. In this context, the information from couples initiating MAP treatment on the risks for the unborn child must be a major concern.
2.2. The issue of genetic screening
Genetic diseases represent one of the leading causes of infant mortality (20%) and are responsible for approximately 20% of hospitalizations in children. Among these genetic diseases, a large number occur through a recessive mechanism (there are more than 3,000 known recessive genetic diseases). In the case of autosomal recessive diseases, the parents, carriers of an altered allele of the “recessive” gene, do not become sick because the second allele is normal. It is the meeting of the two mutated alleles of the same gene that will be responsible, in one in four cases, for the birth of a child affected by the genetic disease linked to the alteration of this gene.
It is estimated that 2% of couples (or 1 in 50) are affected by this risk of autosomal recessive or X-linked genetic disorder, which represents approximately 3,000 births per year in France.
The evolution of genome-wide high-throughput sequencing means that it is now technically possible to screen couples at risk for a large number of recessive genetic diseases. In the current state of knowledge, genetics makes it possible to predict high risks of certain diseases and to develop screening or prevention strategies for predisposed
