*Can environmental influences explain discordance in neuroticism in MZ twin pairs?*

Neuroticism is an important predictor of psychopathology, in particular
emotional (anxiety, depression) and somatization disorders. Genetic studies
generally agree that about half of the variance explaining individual
differences in personality is due to environmental factors. We aim to identify
these environmental factors using a discordant MZ co-twin design. Because MZ
twin pairs share all their genetic material and have been reared in the same
family the method naturally controls for a wide range of potential confounding
genetic and environ-mental factors that are shared by the twin pair. All
phenotypic differences within a MZ twin pair that are not due to measurement
error or random developmental processes must result from differences in
environmental experience(s) specific to the individual twin.

This study aims to identify the specific environmental influences that cause
discordance in neuroticism scores in MZ twin pairs. We hypothesize that the
development of high neuroticism is due to having experienced more severe and
longer lasting environmental adversities throughout life.
Potential causes of discordance are; physical or/and psychological trauma's
or/and apparantly insignificant experiences which cumulate over time or trigger
a particular development that result in persistent changes in neuroticism.

This explorative study will focus on MZ pairs discordant for neuroticism. The
discordant co-twin design will be used, in which genetic and some environmental
confounding is ex-cluded and differences in neuroticism scores are solely
attributable to (nonshared) envi-ronmental influences (and measurement error
and random processes). We will invite the MZ-twin pairs to come for a *twin
research day* at the UMCG-hospital in Groningen. While the first twin has an
interview for about 1,5 hour while the other twin has the time to fill out some
questionnaires. After a break the roles will change. By the use of a
(neuroticism-) discordant co-twin design we will examine and compare the
environments experienced by each individual. We aim to do a retrospective
cohort study, meaning that all the relevant events - life-events, diseases,
accidents e.g., and subsequent development of elevated neuroticism, have
already occurred in the past. We collect the data now, and establish the
likelihood of developing high neuroticism if exposed to particular risk
factors.

The burden associated with the questionnaires and interviews is minimal. Short
breaks will be inserted at the request of the participant, or if the
experimenter feels it is necessary. The duration has been kept in at most 3, 5
hour (90 minutes interview, 90 minutes questionnaires and a debriefing of 20
minutes).