WHICH PATIENT WILL FEEL DOWN, WHICH WILL BE HAPPY? THE NEED TO STUDY THE GENETIC DISPOSITION OF EMOTIONAL STATES Purpose: In quality-of-life (QL) research, the genetic susceptibility of negative and positive emotions is frequently ignored, taken for granted, or treated as noise. The objectives are to describe: (1) the major findings of studies addressing the heritable and environmental causes of variation in negative and positive emotional states; and (2) the major biological pathways of and genetic variants involved in these emotional states. Methods: Literature overview. Results: The heritability estimates for anxiety and depression are 30% - 40%. Related traits as neuroticism and loneliness are also highly heritable. The hypothalamo-pituitary-adrenal axis is the ‘final common pathway’ for most depressive symptoms. The many findings of investigated genes are promising but not definitive. Heritability estimates of positive emotional states range between 40% and 50%. Life satisfaction and mental health share common genetic factors with optimism and self-esteem. The prefrontal cortex is a candidate brain area for positive emotional states. Biological and genetic research into positive emotional states is scarce. Conclusion: Genetically informative studies may provide insights into a wide variety of complex questions that traditional QL studies cannot deliver. This insight in turn will help us to design more effective supportive programs that could moderate the outcomes of genetically-based predisposition.