The Longitudinal Israeli Study of Twins



The Longitudinal Israeli Study of Twins (LIST) is the largest twin study in Israel. It is a longitudinal study that is being conducted mainly in the Social Development Laboratory headed by Professor Ariel Knafo at the Psychology Department of the Hebrew University in Mount Scopus, Jerusalem. The participants in the study are pairs of twins and their families from all over the country. The study deals with the development of children's social behavior and focuses mainly on the development of pro-social behavior: the tendency to help, cooperate, share and be empathetic. As of 2012, around 1600 families have participated in the study.

Being a longitudinal research, this study has followed the twins over a course of a few years in order to examine changes in their development. In the beginning, data was collected on twins from all over Israel that were born in 2004-2005. When the twins turned three, questionnaires regarding the twins' temperament and behavior were sent to their parents. Throughout the study, the families were invited to participate in meetings held at the lab or at their homes in four different time periods: when the twins turned 3.5, 5, 6, and 7. During these sessions, the twins took part in an experiment that was designed to examine their social behavior in a way that would be enjoyable and experiential for them.

The study has two main fields of research:

- Researching the twins themselves: We examine their development, the relations between them and the relations between them and their parents. The study aims to answer questions such as: Are there differences in the development of twins compared to the development of non-twins? Is there dissimilarity between the different kinds of twins in certain abilities or in the quality of the bond between them?

- Researching the development of certain traits and behaviors: The shared genetic heritage of identical and fraternal twins enables us to examine one of the most important questions in Psychology; nature or nurture? In other words, what is the extent of influence of genetic or environmental factors on human development? LIST focuses on researching the development of pro-social behavior and aims to answer questions such as: What is the role of age in pro-social behavior? What is the connection between cognitive abilities and pro-social behavior? What are the critical factors in the development of pro-social tendencies- genetic or environmental factors? How do environmental factors such as parenting and life events work together with children’s genetic tendencies to affect their social development?


What is a twin study?


There are two main types of twin studies.

The first type focuses on the twins themselves, their development and mutual relations. Studies of this sort ask questions such as the following: How do twins develop as opposed to non-twins? Are there dissimilarities between different kinds of twins (identical and non-identical; boy and girl as compared to same sex) in terms of certain abilities or the quality of relations between them? Which processes can parents do, or are already doing, in order to facilitate the twins' proper development, while taking the unique connection between them into account?

Although many twin studies have been conducted over the years, only a few of them have researched the twins themselves and the nature of their mutual relations. Most twin studies are intended to assist the research on development of behavior and different traits by providing answers to one of the most important questions in Psychology: nature or nurture? In other words, what is the extent of the genetic influence as opposed to the environmental impact on human development? The nature-nurture question is investigated through performing comparisons between identical twins (who share 100% of their DNA) and non-identical twins (who share an average of 50% of their DNA). Since the impact of the environment is similar for both identical and non-identical twins, whereas the DNA is identical only in identical twins, these comparisons enable us to learn more about children's development/

One of the unique features of the Israeli Twin Study is that it deals with both kinds of questions, investigating the nature of the twins themselves but also the influence of nature or nurture on development.

Additional fascinating studies are being conducted on identical twins who have been separated at birth and given to adoption. These cases are especially interesting because any differences found between the twins later on in lifecan be attributed solely to the different environments they grew up in (their DNA is 100% identical and thus cannot account for any of the differences). Statistical analysis of these differences enables researches to determine the influence of the genetic and environmental factorson the specific traits that are being investigated.