In TwinLife two different designs will be combined: The cross-sequential design and the Nuclear Twin Family Design (NTFD).
The cross-sequential design is a combination of a longitudinal and a cross-sectional design. While in longitudinal studies the same persons are examined at different points in time (i.e., for instance, via a survey), in a cross-sectional design persons belonging to different age-groups are examined at one certain point in time. When both designs are combined to form a cross-sequential design persons of different age-groups are examined multiple times.
In general, a Nuclear Twin Family Design (NTFD) is an extended twin study design that includes the first-degree relatives of the twins in the assessment. In this case data of the twins, as well as of both parents, and if available of one of the twins’ siblings will be recorded. This is important in order to distinguish the impact of the shared (familial) environment from the impact of the non-shared environment on the development of the twins.
It is important to mention that only twins of the same sex will be surveyed in order to prevent distortions due to gender differences. Moreover, only such pairs will be examined that have grown up or are growing up in the same family.
Tests at home and interviews via telephone will be conducted alternately. This is an advantage because personal contact can be maximized via visits at home. Telephone interviews, on the other hand, reduce the time strain and thereby also lower the drop-out rate.
Through the implementation of the cross-sequential design, TwinLife will observe four cohorts of identical and fraternal twins over a period of twelve years. For this purpose, annual interviews will be conducted alternately via telephone or home visits.
This design enables the examination of a great age range within the short examination period of twelve years. The youngest cohort (born in 2009) will be five years old during the first measurement wave in 2014 and the oldest cohort (born in 1991) will be 31 years old at the last wave in 2023. In consequence, the advantage lies in the fact that the assessment begins shortly before school enrollment and ceases when most of the career-related decisions are completed.
In TwinLife the NTFD will be applied in a special form as the focus will not only lie on the biological family but on the complete environment that the twins grow up in, thus if applicable also including the step-family members. If it is the case that the twins grow up in a step-family or some kind of patch-work family, the biological parents will assessed additionally. Also, separation and divorce of the parents or the moving in of one or more persons into the household will be recorded as these are very important life events. Furthermore, when the twins have reached a certain age spouses will be included in the measurement because they represent a very important environmental factor.