Health care costs tend to be high at the extremes of age and other specific events in life, such as during pregnancy and childbirth. In a national healthcare scheme, these costs are covered by contributions made to the pool over an individual's lifetime (i.e., higher when earning capacity is greatest to meet costs incurred at times when earning capacity is low or non-existent). This differs from the private insurance schemes with contribution rates that vary year by year, according to health risks such as age, family history, previous illnesses, and height/weight ratios.