A new study of more than 1,000 children in New Zealand, following them from birth to the age of 38, found that those who began using cannabis during their adolescence showed a decrease in their general intellectual inability, as measured by IQ tests, from childhood to adulthood.

A person's IQ - or intelligence quotient - does not measure a person's amount of knowledge, but rather represents a person's ability to comprehend concepts, as well their capacity to process information. Typically, IQ does not change significantly over the course of a person's life, unless as a result of severe brain damage from trauma or disease.