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From the age of six, children with a squint are less likely to be invited to a birthday party than their non-squinting peers, a Swiss study suggests.

Researchers showed over 100 children, aged between three and 12, pictures of twins, one with a squint and one without, and asked who they would want to celebrate their birthday with.

Older children repeatedly chose the twin without a squint.

About one in 20 children have a squint.

Also known as strabismus, squint is a condition in which one eye turns inwards, outwards, upwards or downwards while the other eye looks forwards.

Because the eyes focus differently, the brain does not learn to use both eyes to focus together on an object. If one eye is dominant the brain may suppress the image from the weaker eye and control of that eye may be poor.

Squints usually develop during the first three years of life, but can appear later. It is often spotted in early childhood, sometimes within weeks of a baby being born.

Previous studies have shown that those with squints are more likely to suffer prejudice, but this latest study in the British Journal of Ophthalmology appears to pin down the age at which children become conscious of difference.

While children under six made no distinction between the twins with a squint or normally aligned eyes, half of six- to eight-year-old children noticed the squint.

When showed four sets of twins, the majority of children aged between six and 12 never selected a child with a squint or did so only once.

If the squint were not an issue, on average two squinting children per set of four would be selected, the researchers from Kantonsspital in St Gallen believe.

They suggest that all children undergo corrective surgery by the age of six and before "negative social implications may arise".

John Lee, president of the Royal College of Opthalmologists, said in the UK children were treated as soon as was appropriate after they presented.

"But there will be cases where it is better to operate later, and parents advised that we would be best leaving their happy three-year-old alone for the time being.

"What is interesting about this study is that it sets down the age at which their peers become conscious of difference. We don't need to worry too much about children not getting birthday invitations, but should still be aware that there can be very real prejudice."