How similar are twins really?

Sibling myths: Part 3

Jim Lewis and Jim Springer supposedly met for the first time in the Psychology Department at the University of Minnesota. The men had far more in common than just their first names. Both had been married twice before: the first time to a Linda, the second time to a Betty. They had baptised their sons practically identically: «James Alan» and «James Allan». As children, they both had a dog called «Toy».

They had also followed similar career paths, working at times in a petrol station and as a deputy sheriff. They smoked the same brand of cigarettes, both chewed their nails compulsively and had built a bench around a tree in the garden. The famous «Jim Twins» were identical twins and were separated shortly after birth.

Both had been married twice before: the first time to a Linda, the second time to a Betty.

Although they had never met until their participation in the so-called Minnesota Twin Study 39 years later, their lives had been remarkably similar. Thomas Bouchard and his fellow researchers were astonished when they discovered several long-lost pairs of twins with astonishing similarities.

They concluded that genes determine the decisions we make on a daily basis to a not inconsiderable extent. It is true that there are always certain similarities between any two people if you search through all conceivable preferences and characteristics. Nevertheless, the question arises: Is there an invisible bond that connects twins?

In general, the relationship with our siblings is not always unproblematic, but it is one of the most important in life. It outlasts that with our parents and is often even the longest of all. So how close are twins who - if they are born from the same egg - are even genetic doppelgangers?

Greater solidarity

They have a companion of the same age at their side, who grows up together with them, develops at a similar pace, shares interests and passions. And indeed - apparently twins are particularly closely connected.

When Caroline Tancredy and Chris Fraley from the University of Illinois at Urbana-Champaign interviewed adult couples, they were more likely to name their twin as an important attachment figure than other siblings were to name each other.

An indication that they get on particularly well in both senses of the word and also share the highs and lows of life together in the long term.

To get to the bottom of this special relationship, the psychologists asked twins and normal siblings between the ages of 14 and 61 to make statements about the quality of their relationship, for example: «I make an effort to keep in touch with my brother/sister» and «My brother/sister is the person who is always there for me».

Among other things, they wanted to measure closeness and connectedness, components of a strong bond. The test subjects were then asked to rate the extent to which they felt these phrases were true. Once again, the result was the same: twins were obviously closer to each other than other siblings.

Identical twins are closer together

Tancredy and Fraley also investigated what role other potential attachment figures such as father, mother and partner played in the lives of the participants. It was found that the twins surveyed relied slightly less on the support of their parents, friends and even their partner.

Twin couples seem to find something in each other that other relationships cannot offer them. In addition, the relationship apparently intensifies over time. Older twin pairs took on a real bonding function for each other even more frequently than younger ones.

Twins rely less on the support of parents and friends. They seem to find something in each other that other relationships cannot offer them.

The almost symbiotic relationship of some twin pairs is certainly a product of both genetic similarity and a wealth of shared experience. Which factor is decisive can be determined by comparing identical and fraternal twins.

According to this, adult identical twins actually tend to be more closely related to each other than fraternal twins, as Franz Neyer, Professor of Differential Psychology at the University of Jena, demonstrated in 2002.

Genes are more decisive than the environment

In terms of evolutionary biology, this makes perfect sense: if identical twins are particularly close and protect each other, they ensure the continuation of their own identical genes. The security of the bond in monozygotic (identical) twin pairs was not dependent on how often they had de facto contact.

In the dizygotic (fraternal) twin pairs, however, it did play a role in how often they saw each other. The researcher speculates that this may indicate that the ability to form close relationships is subject to a substantial hereditary influence.

In other words, the influence of environment and experience was less significant for identical twins. The bonding of fraternal twins, on the other hand, could depend more on how much they actually invest in the relationship.

Fraternal twins have a deeper relationship than non-twins

Neyer also found that adult monozygotic twins date each other more often and live closer together than dizygotic twins. The study also found that identical twin sisters were closest to each other.

Women with identical genetic material would therefore have the closest relationship. Fraternal male twins brought up the rear - although mixed-sex couples were not considered.

Researchers found that twins of the same sex argue more often.

A similar pattern can already be found in young children. Psychologists from the Hebrew University of Jerusalem explored the relationship between three-year-old twins and siblings by asking mothers about closeness, dependency, rivalry and conflicts between their offspring.

As it turned out, the fraternal twins had a deeper relationship than non-twins - a possible indication that growing up together at the same age brings siblings closer together.

Twins of the same sex were ahead in arguments and competition. They may have to fight harder for their parents' attention as they have very similar needs and are also more likely to be compared with each other.

As expected, identical twins were even closer to each other than fraternal twins and were also more dependent on each other. The researchers found that the children reacted sadly according to their mother as soon as they were separated or the other was ill or upset.

«Since identical twins tend to have a closer relationship than fraternal twins, who, like normal siblings, only have 50 per cent of their genetic material in common, genetic similarity must have a decisive influence on the quality of the relationship,» says Rainer Riemann, Professor of Differential Psychology at Bielefeld University.

«We know from studies that the death of the other is more drastic for identical twins than for other siblings. They also suffer more when there is conflict with the other. This may be due to the thought: if we are so similar, something must have gone fundamentally wrong here.» Biological kinship therefore plays an important role in social relationships.

People who are so genetically similar are also very likely to have similar personalities, share key attitudes and interests, end up in comparable life situations and are therefore more likely to be on the same wavelength. These similarities form the basis for the special relationship between identical twins.

This article first appeared in «Spektrum Psychologie», 4/2018