Children who grow taller, grow smarter - and scientists now believe they know why. Researchers at the University of Bristol have shown that there is a link between a child's IQ and the level of growth hormone circulating in their blood, which determines the speed at which they arrive at their final adult height.

This could explain why some shorter children do worse at school. Taller children often have higher IQs and short children treated with growth hormone have seen their IQs improve.

The study involved 547 children who completed an intelligence test at the age of eight. The researchers then measured the level of insulin growth factor (IGF) in their blood. Higher levels of IGF matched higher IQs.

Professor David Gunnell, who led the study, said: "Poor foetal and post-natal growth are associated with impaired neurodevelopment. Low birthweight babies experience delays in reaching motor milestones and on average have slightly lower IQs than babies of normal weight."

IGF plays a key role in physical growth and organ development during childhood. The level of IGF in the blood is influenced by diet. Children who drink more milk and eat more dairy produce in early childhood have higher levels.

Professor Gunnell added: "Short stature - a measure of poor post-natal growth and nutrition - is associated with low scores in tests of cognitive function and poor educational achievement."The findings are published in the US journal, Pediatrics.