Being bottle-fed as a baby makes you more likely to be left-handed, research has found.
Debate has raged over whether nature or nurture decides our dominant hand.
Even identical twins – who have the same genetic code – differ in whether they are left-handed or right-handed.
The new research says that nurture plays a decisive factor.
For around one in five left-handers, the key reason for their dominant handedness is whether they were breast fed or not.
The study was based on research in five countries – including in the United States, the United Kingdom, and Ireland – involving 62,129 pairs of mothers and babies.
Breastfeeding for more than six months overall decreased the chance of any baby being left handed from 13 per cent to 10 per cent, the researchers said.
This means that around 22 per cent of left-handers would be right handers if they had been breast fed for six months or more, rather than bottlefed.
The researchers suggest that the process where the right side of the brain and left side of the brain take on specialist functions takes place up to nine months.
Advantages for babies who are breastfed by healthy mothers include increased intelligence, increased head circumference, decreased speech problems, and decreased multiple sclerosis, the researchers said.
By: Colin Ferguson, Science correspondent for the daily mail