A new study in the Department of Psychology at the Hebrew University sheds light on the complexity of the relationship between twins, and also examines how parenting affects these relationships. This is the most comprehensive study done regarding the relationship between twins throughout childhood

Relationships between twins have fascinated the human imagination since the dawn of history. Jacob and Esau, Castor and Pollux, Remus and Romulus, represent only some of the examples of dealing with the relationship between twins. The twins have a central influence on many characteristics in the lives of the twins and their families, among others regarding mental health, behavioral problems, ability to create social relationships, education and professional development. While the birth rate of twins has been increasing in recent decades, and in Israel one out of 10 children is a twin, academic knowledge about relationships between twins is scarce, and very little is known about the complexity of relationships between twins.
A new study, which examines in depth the relationship between twins throughout childhood and is considered the most comprehensive study on this topic, was conducted by Dr. Hila Segal and Prof. Ariel Kanafo-Noam from the Department of Psychology at the Hebrew University and was published in the Journal of Social and Personal Relationships. The researchers examined the relationship the twins throughout their childhood with the aim of understanding what the main dimensions of these relationships are, how the different dimensions of relationships develop over the years, and also how parenting style affects the nature of the relationships of the twins
The study was conducted as part of the Israeli twin study, headed by Prof. Knafo-Noam, and twins and their families from all over the country participate in it. The study sample included over 1500 twin pairs and their families and the researchers followed them from age 3 to age 9. For the purpose of the study, the parents of the twins answered the questionnaire on the relationship between the twins and parenting questionnaires. The zygosity of the twins (being identical twins or fraternal twins, i.e. non-identical) was determined based on DNA tests and parental questionnaires. The research method included Latent Growth Modeling, which describes the development of the twin relationships throughout childhood, the connections between the different relationship dimensions, and the effect of zygosity and parentage on these relationships.
The researchers found five main dimensions that exist in the relationship between twins: closeness (for example, "Having a lot of fun with the other twin"), dependence (for example, "gets angry when he is apart from the other twin"), dominance (for example, "likes to tell the other twin what to do") , conflict (for example, "fights and argues with the other twin") and rivalry (for example, "jealous or not happy when [the mother] does things with the other twin"). It was found that throughout childhood the twins' relationship is characterized by high levels of closeness and low levels of conflict. While there has been a slight decrease in the dimensions of closeness and dependence between the twins, the level of conflict between them remains fairly constant. According to the researchers, "the understanding of the complexity of the twin relationship can be used by parents and caregivers when they come to raise and care for twins. For example, parents may be concerned that the fights between the twins will affect their degree of closeness, but we found in the study that the two dimensions, conflict and closeness, are not related to each other, and can exist at the same time."
Also, the researchers found that the level of rivalry increases around the age of 6, but decreases thereafter. Dr. Segal: "The rivalry between the twins is related to the competition between them. We hypothesize that entering school at the age of 6 increases the rivalry between the twins because a comparison between them receives formal signs. This is the first time that there are clearer criteria for comparison, based on grades, progress in reading and arithmetic, etc.
While previous studies have found that identical twins are closer to each other than fraternal ("non-identical") twins, the current study indicates that the close bond begins in early childhood. In addition, while in fraternal twins closeness decreases throughout childhood, closeness between identical twins remains stable. The research also shows that the level of conflict and rivalry between identical twins and fraternal twins is similar. According to Dr. Segal: "The finding that genetic similarity affects kinship relationships but not conflict relationships is important for understanding the complexity of twin relationships. We hypothesize that the great closeness between identical twins relies on an evolutionary mechanism of promoting shared genes. On the other hand, the competition for the "parental resource" remains the same even among identical twins, and creates conflicts in their relationships as well as between fraternal twins.
Another interesting finding is related to parenting. It was found that positive parenting (warm and supportive) is related to close relations between the twins, while negative parenting (hostile and punitive) is related to conflict between the twins. Moreover, negative parenting predicts an increase in conflict between the twins throughout childhood. Prof. Kanfo-Noam: "This finding can guide parents in creating more positive relationships between their twin children and in understanding their power and influence on relationships."
The story of Jacob and Esau emphasizes the rivalry between the twins from their mother's womb, through the story of their first birth and the separation between them that lasted 20 years, until the exciting meeting between them that led to the renewal of the relationship. The adult Jacob's fear of the meeting was so great - that he preferred to divide the entire camp he had in two. But the event ended well: "And Esau ran to call him and embraced him, and he fell on his neck and they watered him; and they wept." Dr. Segal explains: "The current study shows how the twin relationship is not one-dimensional - not only closeness but also conflict, not only dependence but also rivalry. Therefore, although the biblical author chooses to describe to us the rivalry between Jacob and Esau, fraternal twins, it can be assumed that the rivalry did not prevent them from having moments of mutual closeness between them as well."
Dr. Segal concludes and claims that "one of the questions that bothers parents, educators and caregivers is the issue of separating twins in educational settings. Our findings show that it is not possible to give a general recommendation regarding the separation between twins, and that it is worthwhile to examine the complexity of these relationships specifically between each pair."
For the scientific article: https://onlinelibrary.wiley.com/doi/full/10.1111/acel.13386
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