The history of emotions has become an increasingly popular topic recently, with some scholars arguing that it is an essential category of analysis, not unlike class, race, or gender. Historians, like other social scientists, assume that emotions, feelings and their expressions are regulated in different ways by both different cultures and different historical times, and constructivist school of history claims even that some sentiments and meta-emotions, for example Schadenfreude, are learnt and not only regulated by culture. Historians of emotion trace and analyse the changing norms and rules of feeling, while examining emotional regimes, codes, and lexicons from social, cultural or political history perspectives. Others focus on the history of medicine, science or psychology. What somebody can and may feel (and show) in a given situation, towards certain people or things, depends on social norms and rules. It is thus historically variable and open to change. Several research centers have opened in the past few years in Germany, England, Spain, Sweden and Australia.