TwinLife sets out to take a look at the biological origin as well as the social origin of social inequality (Figure 1). Using the data of identical and fraternal twins as well as their families, the impact of genetic differences on certain behavior can be determined. Furthermore, environmental characteristics such as the socio-economic status (SES), family structure and home environment, relations among family members as well as characteristics of the neighborhood are observed.

 
Figure 1. Basic concept and six domains of social inequality.

Social inequality is divided into six areas (see Figure 1) that reach from education and labor market attainment to health, psychopathology and deviant behavior.
I.    Skill formation and education
This area contains the motivation and level of education in addition to cognitive abilities and their development.
II.    Career, labor market attainment, and welfare
This domain includes the perceived security of the workplace, job satisfaction, commitment to work, current occupation and current position as well as income or, if applicable, the receipt of social benefits.
III.    Political and social integration and participation
The characteristics of the social environment of every person such as the support by family, friends and spouses are covered in this area. Furthermore, social and political commitment and social resources are of particular interest.
IV.    Subjective perception of quality of life
This category comprises, e.g., a person’s self-esteem, global life satisfaction as well as their satisfaction concerning specific areas.
V.    Physical and psychological health
Assessments of general health in terms of diseases, but also subjectively perceived impairments as well as information on health behaviors were covered in this area.
VI.    Psychopathology and deviant behavior
Criminal or delinquent behavior as well as the degree of internalizing and externalizing problem behavior are surveyed in this domain.

In order to take a closer look at the emergence of social inequality, TwinLife not only looks at different causal factors, but also different indicators of personal and social success or failure. Figure 1 provides an overview of the relevant characteristics. On the side of the determining factors (left), both genetic predispositions and aspects of the environment in which children and young people grow up and live are taken into account. On the impact side (right), not only objective but also subjective information about a person's individual career is considered. For example, the educational path and entry into work, as well as participation in social, cultural and political life and the subjective assessment of quality of life are examined. Children are influenced by their environment, but they can react differently to this environment depending on their individual characteristics. They can also interact with it and design it. In order to be able to take a closer look at this reciprocal process, the characteristics shown in Figure 1 should be recorded over a period of 8 years. This makes TwinLife broader than most comparable twin studies and should be understood more like a classic social science study. In this way, many areas of life, outcomes and possible origins of social inequality are examined. Figure 1. Basic concept and six domains of social inequality.With the help of information from monozygotic and fraternal twins, the significance of genetic differences for numerous development indicators such as behavior can be determined. In addition to biological characteristics, environmental characteristics are also considered, such as socio-economic status, family environment and structure, relationships between family members and neighborhood characteristics. Certain genes in a person can be “switched on” or “muted” by environmental influences. This, e.g., plays a role in the development of diseases and can explain why identical twins can have the same genetic makeup for Alzheimer's, but not both develop it. We investigate epigenetic interactions in our satellite project TECS. “Person” means, on the one hand, the individual and his behavior and, on the other hand, their characteristic behavioral tendencies, which in psychology are referred to as “personality”. A person's personality influences their behavior in almost all areas of their life. The “Big Five” model can be used to provide a comprehensive description of personality. It includes, for example, the characteristics of extraversion, openness, or conscientiousness. But also other features, such as self-efficacy perception, self-confidence, or dealing with stress (so-called coping styles) play a role. The environment includes all external influences. This includes, for example, whether you grow up in the city or in the country, the area where you live, which school or place of work you attend, or the income that a family has at its disposal. The special importance of environmental influences is taken into account in the satellite project “TwinLife Environment”. This is about the fact that the environment and the person influence each other, that is, they interact with each other. On the one hand, the environment affects the people in it - but at the same time the people also create the environments around themselves and for others. This could be the case, for example: An extroverted person creates an environment with lots of friends, or growing up on a farm makes a person more likely to love animals. Culture in the broader sense (see “social and cultural transmission”) also plays a role. The social and cultural transmission of social inequalities describes the shaping of people's perception and behavior by socially and culturally shaped norms and values. These influence the opportunities, behavior and ultimately the experiences of people within society. They can thus contribute to the creation or maintenance of social inequalities.Social inequality can be described from different perspectives that reach from education and labor market attainment to health, psychopathology, and deviant behavior.This area contains e.g., the level of education, educational aspirations, and achievement motivation in addition to cognitive abilities and their development. This domain includes the perceived security of the workplace, job satisfaction, commitment to work, current occupation, and current position as well as income or, if applicable, the receipt of social benefits. The characteristics of the social environment of every person such as the support by family, friends, and spouses are covered in this area. Furthermore, social and political commitment and social resources are of particular interest. This category comprises, e.g., global life satisfaction as well as their satisfaction concerning specific areas, life goals, and subjective perceptions of inequality. Assessments of general health in terms of diseases, but also subjectively perceived impairments as well as information on health behaviors are covered in this area. Criminal or delinquent behavior as well as the degree of internalizing and externalizing problem behavior are surveyed in this domain.