Almost half of all preschoolers don't play outdoors anymore, a new US study has found.

The study, published in the journal of Archives of Pediatrics & Adolescent Medicine, found that those children whose parents did not rely on other forms of childcare other than themselves did not play outside or run around in a park daily – representing almost 50 percent of the sample group.

This result was regardless of factors such as the income of the household, or whether or not parents had their children watch TV too often. Instead, the result seemed most strongly associated with how active the parents were, how many playmates the child had and even the sex of the child.

Researchers found that boys were more likely to play outside more often than girls, as were youngsters with more friends and more active parents.

The study was conducted by analysing data from the habits of 8950 preschoolers (age ranging from three to four years old) from a sample group representative of the US population.

Reduced outdoor play can have a major impact on a child’s motor development and good mental health.

Outdoor play can improve bone growth through the production of Vitamin D, encouraged by sun exposure and can even reduce the risk of developing myopia, according to a 2011 study.

Researchers have noted a steady decline in play time. A 1989 study found that 96 percent of primary schools had at least one break time during the school day. In 1999, only 70 percent of preschool classrooms had break time in which the children are allowed to play freely outdoors, according to the National Association of Elementary School Principals.

The American Academy of Paediatrics recommends children be encouraged to spend most of their time in play and that those younger than two should avoid watching television as much as possible.