A study of U.S. death certificates reveals that people with Down's syndrome are one-tenth as likely to get cancer than the rest of the population.

The statistical study also shows the life expectancy of people with the genetic disorder has doubled in the last two decades.

Quanhe Yang of the U.S. Centers for Disease Control and Prevention said the result was unexpected and more research is needed to determine why Down's syndrome protects against cancer.

The analysis of 17,900 American death certificates from 1983 to 1997 showed the average age of death of people with Down's syndrome increased from 25 to 49.

It also showed that cancers other than leukemia and testicular cancer were listed on their death certificates one-tenth as often as expected.

The study appears in Friday's edition of The Lancet.

Down's syndrome is a genetic disorder that occurs in about one in every 800 births. About 42,000 have the condition.

People with Down's syndrome have an extra copy of the 21st chromosome, which causes mental retardation, thyroid problems and heart defects.

Yang put forward three possible explanations for the lower risk of cancer death in people with Down's syndrome:

  • their extra copy of the 21st chromosome could contain extra tumour-suppressing genes
  • their cells could have a slower rate of replication or a higher rate of cell death, meaning cancer tumour would grow more slowly
  • they don't tend to smoke or drink heavily, so they are exposed to fewer environmental causes of cancer