that first-borns were first spoiled and then “dethroned”, which would constitute a form of trauma. In order to overcome this trauma, they would reinforce the imitation of their parents and the feeling of responsibility towards the younger children. They would thus put more emphasis on rule, discipline and become more conservative. First-borns who do not overcome this battle for the “lost love” of the mother become angry or resigned.
For a long time, a different hypothesis was also assumed, that for first-borns, parents are inexperienced and are more anxious and more demanding in their parenting behavior. This would make the older children more fearful and more dependent (and possibly therefore more dependent on psychoactive substances?). The younger children would be more open, more independent and more rebellious, and the older ones would be more conformist, more ambitious, etc. The feelings of an only child would be closer to those of an older child than a younger one.
However, what do 50 years of research on sibling rank show? Well, they show that there is no correlation between sibling rank and personality, in particular, that we do not find the above associations with openness of mind. In fact, the results of the studies go in all directions, and analysts have not been lacking in imagination when explaining completely opposite results, based on the use of popular theories about the effects of rank, when it suits the reasoning.
Associations with the subject’s personality traits are only found in descriptions by parents or siblings. In fact, the influence of sibling rank would only be subjective: the way others see you, the way you behave at home because “an older child is supposed to behave like this or like that”, but in no case does this translate objectively into measurements when taking personality tests.
1.14. Sibling size
There is a stereotype of the only child (egocentric, tyrannical, etc.), but does this cliché correspond to reality? Studies on this topic have not found any particular characteristics of only children (Almodovar 1981). It is true that childhood experiences are different for only children, but we can imagine that relationships with parents and then with peers would create a strong psychological variability among these children.
In China, the one-child policy in particular led to the belief that these only children were “little emperors”, corresponding to the stereotype.
Male only children received fewer positive evaluations than female only children. In particular, they were found to be at greater risk of obesity.
Evaluations of only children are better after the implementation of the one-child policy in 1979 than before, but the policy has just changed.
1.15. Twins
The birth of twins is always a phenomenon that arouses curiosity. It is known that there are two types of twins: dizygotic twins resulting from the fertilization of two ovocytes by two different spermatozoa, and therefore have a different genetic heritage, like brothers and sisters; and monozygotic twins resulting from the division of a single embryo and therefore have an identical genetic heritage.
The prevalence of twin births has increased over the past 40 years in France:
– 1972: 9 births out of 1,000;
– 2000: 15 births out of 1,000;
– 2013: 17 births out of 1,000.
This increase is mostly related to dizygotic pregnancies, the prevalence of monozygotic pregnancies being stable at around 1 to 3 per 1,000 births.
Prevalence varies according to culture: in Europe and the United States, the rates are between 10 and 20, in Asia 6 and in some African countries 40.
Why this increase? Two factors have been put forward:
– the increase in the age of the mother at first child, that we mentioned above;
– the developments in medically assisted procreation. Thus, the proportion of twin births is 17% following in vitro fertilization, 9% following intrauterine insemination with the spouse’s sperm and 11% with donor sperm.
1.15.1. Aspects of vulnerability
Twins have greater morbidity and mortality than other children:
– from the fetal period: intrauterine growth retardation (average weight of twins under 600 grams), TOPS (Twin Oliguria Polyhydramnios Syndrome, or acardiac twin, an anemia related to hemodynamic and hormonal imbalance in 5 to 15% of cases), congenital malformations (mainly heart disease, relative risk of 1.7), prematurity (in 43% of cases, or 7 times more frequent than for single pregnancies), perinatal anoxia (especially for the second twin);
– higher neonatal mortality: in the EPIPAGE cohort study of low gestational ages (2,773 premature infants, before 32 weeks), the risk of death in hospital was increased for twins, after adjustment for gestational age and sex, and their survival at five years was decreased. Some factors influence this mortality, such as birth order or weight discrepancy (relative risk of 2.9 in the EPIPAGE study);
– sudden infant death syndrome has been observed to be more frequent in twins, but this correlation no longer holds when birth weight, prematurity and sociodemographic data are controlled.
These increased neonatal and perinatal risks in multiple pregnancies are no longer observed later in life (cancers and cardiovascular diseases are not more prevalent in twins).
Regarding development during childhood: there is no delay in psychomotor development according to the EPIPAGE study; there is no difference in IQ when prematurity and birth weight are controlled.
1.15.2. How can we explain the differences between two monozygotic twins?
Monozygotic twins are described as identical, but this is a simplification and there are usually differences between them.
These differences may come from:
– genetic factors (alteration of genes by DNA mutations, or of chromosomes by postzygotic mitotic errors, polymorphisms);
– epigenetic factors (differential DNA methylation and histone modifications, X chromosome inactivation, genomic imprinting);
– mitochondrial inheritance (mitochondrial DNA mutations);
– differences in intrauterine environments.
1.15.3. The psychological consequences of twinning
1.15.3.1. On parents
Multiple pregnancies can, from the start, be a risk factor for psychological difficulties and ambivalent attitudes.
Depression is five times more common in mothers of twins: the care and housework associated with the arrival of twins has been estimated at 12 hours per day, resulting in sleep and time deprivation. Hospitalization of one or both twins may also delay attachment mechanisms.
Concerning the father, his help during a multiple birth is often important at the beginning, but decreases with time.
1.15.3.2.
