Social attitudes towards children differ around the world in various cultures. These attitudes have changed over time. The age at which children are considered responsible for their own actions (e.g., marriage, voting, etc.) has also changed over time, and this is reflected in the way they are treated in courts of law. Child development refers to the biological and psychological changes that occur in human beings between birth and the end of adolescence. There are various definitions of periods in child development, since each period is a continuum with individual differences regarding start and ending. Some age-related development periods and examples of defined intervals are: newborn (ages 0–1 month); infant (ages 1 month – 1 year); toddler (ages 1–3 years); preschooler (ages 4–6years); school-aged child (ages 6–13 years); adolescent (ages 13–20). Surveys have found that at least 25 countries around the world have no specified age for compulsory education. Minimum employment age and marriage age also vary. In some countries, children are legally obliged to go to school until they are 14 or 15 years old, but may also work before that age. A child's right to education is threatened by early marriage, child labour and imprisonment. The right to education is recognized as a human rights commission to establish an entitlement to free, compulsory primary education for all children, an obligation to develop secondary education is also accessible to all children, as well as equitable access to higher education too, and a responsibility to provide basic education for individuals who have not completed primary education.