Human development can be understood as an interplay between biological, socio-historical and social conditions, as well as individual developmental actions. Personality is a prominent psychological concept that is unquestionably understood as being individual. Today’s developmental psychology assumes that a person’s personality is unique, co-determined by biological processes (nature), socio-historical and social conditions (nurture) and shaped by developmental actions (individual), and increasingly stable across the life span. This project investigates a new question, i.e., the relative dependence of personality development in a couple during a major life transition, i.e., becoming for the first time a parent. We label this process intra-generational co-development of personality. The next question concerns co-development in personality from an inter-generational perspective. The first inter-generational dependence is the baby and the second concerns the parents’ parents, i.e. the grandparents. The questions will be investigated within a longitudinal multicenter multigenerational study design (study sites in Basel, Zurich and Lausanne). This initial project funded by the Research Fund of the University of Basel is intended to result in a full research proposal to be submitted to the Swiss National Science Foundation (SNSF) within the SINERGIA programme. This initial project has for purposes, i.e., (1) to carefully study the literature and to condense the literature in the full research proposal to submit to the SNSF, (2) to definitely outline the study design, (3) to conduct a feasibility study and (4) to prepare the application for the ethical approval of this study. In parallel, the co-investigators (Proff. Grob uniBAS, Krings uniL, Martin uniZH) prepare the specific research questions for their groups and start Master-and PhD-projects in the respective domains at their sites.