We propose a new hypothesis – Selective Outmigration by Personality Hypothesis (SOPHy) – to explain how Scandinavians have come to exhibit the highest levels of subjective well-being in the world. We assume that Viking men might have been similar in personality to modern violent and property criminals, who are low in Conscientiousness and Agreeableness and high on Neuroticism. Since less Conscientious and Agreeable and more Neurotic individuals are on average less happy, their selective outmigration from Scandinavia during the Viking Age (793–1066) could have elevated the genetic tendency to be happy among Scandinavians left behind. Our calculations show that genetic selection alone could have produced the current Scandinavian genetic advantage in happiness under reasonable assumptions. The same process may also explain the exceptionally low level of happiness among Russians, who descended from Vikings. The Viking Age was a rare historical event in selective outmigration by personality not shared by other examples of mass outmigration, such as Genghis Khan’s army, the Crusades, and penal colonies in Australia. Other examples of SOPHy might include the emigration of Japanese laterborns (birth order qua personality) and the Irish Potato Famine (occupation qua personality). Our hypothesis awaits empirical testing with historical, archeological, and population genomic data.