This research focused on the relationship between risk variations of dopamine genes as well as the serotonin gene and executive functions. To conduct a series of meta analyses, several electronic...Show moreThis research focused on the relationship between risk variations of dopamine genes as well as the serotonin gene and executive functions. To conduct a series of meta analyses, several electronic databases concerning these genes were consulted. The analyses encompass the following dopamine genes: COMT, DRD4, DRD2, DAT1 and MAOA. It also encompasses the serotonin gene 5-HTTLPR. The executive functions that will be discussed are inhibition, cognitive flexibility, working memory, planning, and attention. All studies were encoded independently by two coders. The variables are age, sample sizes, DNA deduction, category of the construct’s result (which is part of the executive functions), measuring methods, and the genes with the accompanying polymorphisms. The genes COMT and DRD4 showed relationships between the risk variations of these genes and the executive functions. People with the risk variation of the COMT and the DRD4 genes performed less well when it came to the executive functions, compared to people who did not have this variation. In addition to this, age is a moderator in the relationship between polymorphisms of the COMT gene and the executive functions. The difference between the Met-Met variant and the Val-Val variant of the COMT gene was larger when it came to the executive functions concerning people of twenty-one years old and older; the difference is smaller for people younger than twenty-one. The genes DRD2, DAT1, MAOA and 5-HTTLPR did not show any relationship between the risk variations of these genes and executive functions. Heterogeneity and publication bias could have restrained the relationships between dopamine genes and the executive functions. Throughout this research the perspective of the theory of differential susceptibility will be maintained. Also, this research will illustrate how the results are contributing to science, and how they are useful in practice.Show less
Recent years several studies have attempted to examine the associations between polymorphisms of COMT, DRD4, DRD2, DAT1, MAOA, 5-HTTLPR, and executive functions. These studies have never been...Show moreRecent years several studies have attempted to examine the associations between polymorphisms of COMT, DRD4, DRD2, DAT1, MAOA, 5-HTTLPR, and executive functions. These studies have never been combined in a meta-analysis. In this thesis a meta-analysis is carried out for each gene separately. This leads to a central research question: Do people with risk variants of dopamine genes or the risk variant of 5-HTTLPR perform worse on executive function tasks than people without these risk variants? Several databases were searched for relevant studies relating the polymorphisms of COMT, DRD4, DRD2, DAT1, MAOA, and 5-HTTLPR to executive functions. This resulted in 23 studies in the meta-analysis of COMT, 10 studies for DRD4, 6 for DRD2, 9 for DAT1, 4 for MAOA, and 9 for 5-HTTLPR. The meta-analyses only included studies with a non-clinical sample. Significant associations between polymorphisms of COMT and DRD4 and performance on executive function tasks were found. Both set of studies were however not homogene, so these results must be interpreted carefully. Also, a moderator analysis for COMT was carried out to examine whether a difference could be found between the effect size of cognitive flexibility and the effect size of other executive functions. The results showed that the polymorphism of COMT was significantly more associated with cognitive flexibility than with other executive functions. Despite several limitations of this study regarding the overlap between executive functions and the disadvantages of meta-analytic techniques, this study contributes to more understanding of the association between genes and executive functions.Show less
