Sisu is a Finnish concept held by Finns themselves to express their national character: it is extraordinary determination in the face of extreme adversity, and courage that is presented typically in situations where success is unlikely. It expresses itself in taking action against the odds, and displaying courage and resoluteness in the face of adversity; in other words, deciding on a course of action, and then adhering to it even if repeated failures ensue. Stoic determination, tenacity of purpose, grit, bravery, resilience, and hardiness help describe Sisu. Sisu is a term which dates back hundreds of years and is described as being integral to understanding Finnish culture. It is a term for going beyond one’s mental or physical capacity, and is a central part of the country’s culture and collective discourse. Despite Sisu being studied as a cultural component among Finns and Finnish Americans, hardly any empirical research has been done to explore the meaning of this construct as a possible psychological strength resource, therefore as a psychological construct it has remained under-researched and poorly defined. Like any trait or psychological capacity, sisu is the complex product of genetic, psychological, biological and social factors, and its comprehensive understanding will require studies from multiple scientific perspectives. Research on sisu was started in 2013 (currently continued on the Ph.D. level at Aalto University School of Science in Espoo, Finland) to render it less elusive as a construct by giving it an easily citable definition rooted within the field of positive psychology: in the study, sisu is described as a psychological key competence which enables extraordinary action to overcome a mentally or physically challenging situation. Sisu also contributes to what has been named the action mindset; a consistent, courageous approach toward challenges which at first seem to exceed our capacities. Sisu is still deeply valued among contemporary Finns (and Finnish Americans) and revealed the perception of sisu as a reserve of power, which enables extraordinary action to overcome mentally or physically challenging situations (rather than being the ability to pursue long-term goals and be persistent). To elaborate on the function of sisu: it is a psychological potential that enables the individual to tap into strength beyond their pre-conceived resources. Wielding sisu in the face of adversity helps individuals push through what first seemed like the boundaries of their mental or physical capacities. Furthermore, sisu is an action mindset that equips the individual to choose to take on challenges beyond their observed capacities. It provides the final empowering push when we would otherwise hesitate to act. Sisu can be conceptualized as taking action against the odds. Sisu is also a flexible quality that can be cultivated through conscious practice, rather than being a fixed quality. Sisu is not always an entirely positive quality: in Finnish, ‘pahansisuinen’ literally translated means ‘one possessing bad sisu’, a description of a hostile and malignant person. Also, there can be too much sisu, which may lead to bull-headedness, foolhardiness, self-centeredness and inflexible thinking: sisu should be informed by reason and cultivated (and practiced) with self-compassion. Finland may have the initial monopoly on sisu as a cultural construct, but it is a universal capacity and the potential for it exists within all individuals. The transformative power of narrative is widely acknowledged: through the process of social transfer of narratives, values become embedded within a culture and connected to the thought processes of its individuals. People, through their choices and actions, bring these narratives to life in a dynamic process which can prime the behavior of an entire nation. Fostering sisu may very well be embedded in such behavior, rather than being a genetical trait which one is born with. Sisu is a new term in the field of positive psychology, and it may contribute to our understanding of the determinants of resilience, as well as of achievement and the good life. It is suggested that positive psychology research could benefit from focusing future interest on the unique cultural resource of sisu that individuals across the globe can leverage; as well as actively examining relevant constructs from other cultures. Read more: Sisu on Wikipedia