'Retard' is a now derogatory term for mental retardation, a generalised intellectual developmental disorder evident in childhood typically defined as an IQ score under 70. 'Retard' was originally used as a medical term but became a term with pejorative connotations and has been superseded by terms including 'special', 'challenged', 'intellectually disabled' and 'developmentally delayed'. Joe has Down's syndrome, a condition caused by an extra 21st chromosome named after John Langdon Down, the physician who first described the condition in 1866. Down's syndrome occurs both because of genetics and by pure chance, although older parents are more likely to have a child with the condition. The average IQ of individuals with Down's syndrome is 50 compared to 100 in the general population and the condition has some common physical manifestations. Intellectual capabilities and physical differences are still varied amongst individuals and in recent decades societal expectations of people with Down's syndrome has significantly increased. Individuals with the condition have a higher risk of other medical conditions (especially heart defects), which result in often decreased lifespans. Although the average lifespan in the United States in 2002 was 49 years, this is a significant increase from 25 years in 1980.