Pooling already collected data sets offers a shortcut for international comparisons. We combined this approach with the twin study design to analyze how the societal macro-environment modifies genetic and environmental variation. For practical reasons, we targeted simple traits representing anthropometrics (height and weight), socioeconomic (education), and health behavior (smoking). The key principles of the project are that our collaborators can decide in which specific analyses their data can be used and that all scientific merits will be shared fairly with all collaborators. We first identified 69 twin cohorts in the world and asked the principal investigators of them to provide data to this international project. After three reminders, 51 twin cohorts representing 23 countries contributed data to this project. We first harmonized the anthropometric and background variables and, after publishing the first research articles, harmonized the education and smoking variables. Working with multiple cohorts and big research groups creates challenges, but is also rewarding. The possibility to compare different countries creates a totally new dimension for research, facilitating the use of already collected data. International comparison creates new knowledge not possible to achieve without collaboration, even when using simple traits. Our project shows that collecting an international database is possible with limited financial resources if utilizing already collected data.