Ageing (British English) or aging (American English) is the process of becoming older. It represents the accumulation of changes in a person over time. In humans, ageing refers to a multidimensional process of physical, psychological, and social change. Reaction time, for example, may slow with age, while knowledge of world events and wisdom may expand. Ageing is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions. Ageing is among the largest known risk factors for most human diseases. Roughly 100,000 people worldwide die each day of age-related causes. Population ageing is the increase in the number and proportion of older people in society. Population ageing has three possible causes: migration, longer life expectancy (decreased death rate) and decreased birth rate. Ageing has a significant impact on society. Young people tend to have fewer legal privileges (if they are below the age of majority), they are more likely to push for political and social change, to develop and adopt new technologies, and to need education. Older people have different requirements from society and government, and frequently have differing values as well, such as for property and pension rights. Recent scientific successes in rejuvenation and extending the lifespan of model animals (mice 2.5 times, yeast and nematodes 10 times) and discovery of variety of species (including humans of advanced ages) having negligible senescence give hope to achieve negligible senescence (cancel ageing) for younger humans, reverse ageing, or at least significantly delay it. Regenerative medicine is a branch of medicine associated with the treatment of age-related diseases. Ageing is the major cause of mortality in the developed world.