Sign language may be behind rising rates of inherited deafness, according to researchers.
A mutation in one gene causes 50% of inherited deafness
The proportion of people who are born with profound hearing loss has doubled over the past 200 years.
Researchers at Virginia Commonwealth University in the United States have traced the increase back to the introduction of sign language in the early 1800s.
They say the introduction of sign language allowed people who are deaf to communicate with each other more easily.
They say it also led to many more people with hearing loss marrying.
The researchers believe the decision of so many people with inherited hearing loss to marry has been behind the increase in deafness rates.
More than 100 genes are involved in hearing loss. As a result, most people who are deaf have children with normal hearing because they pass on different genes.
However, a mutation in one gene, the connexin gene, is responsible for 50% of inherited deafness.
If both parents have this mutation, they will pass it on to their children, who will usually be born deaf.
The researchers used computer modelling to show what effect intermarriage between people with inherited deafness has over many generations.
They found that intermarriage between people who are deaf can lead to a significant increase in the number of people with inherited hearing loss.
This is largely because they have children with the condition who themselves go on to marry other people with this type of genetic deafness.
"In the United States, at least 85% of individuals with profound deafness marry another deaf person," said Professor Walter Nance, who led the study.
"In the case of marriages among couples who both have the same form of recessive deafness, all their children will be deaf and capable themselves of also passing on the altered gene to their offspring.
"In addition, as many as 3.5% of the hearing population in the United States may carry single mutations involving the connexin 26 complex, making this one of the most commonly recognized single gene defects."
Professor Nance said areas in the United States with a history of schools that teach through sign language have increased rates of genetic deafness.
"In regions where national or state-wide schools for the deaf have been established and marriages among students have occurred, we've seen an amplification of the commonest form of recessive deafness in the overall population."
Professor Nance believes the findings may explain how speech evolved in humans.
"When you think about how the onset of selective marriages among deaf populations led to an increase in specific mutations for deafness, you easily can see how these same forces might have contributed to the spread of genes for speech among Homo sapiens 160,000 years ago," he said.
"If you were one of the first primates with an ability to communicate by speaking, wouldn't you want to select a partner who could whisper sweet nothings in your ear?"
Brian Lamb, director of communications at the RNID, said:
"This research provides an interesting insight into why a specific genetic form of deafness has become more common.
"It is not surprising that people with a shared culture of deafness marry. And if both people have the same specific genetic cause of deafness, this will be passed down the generations.
"RNID supports the rights of deaf people to marry one another. As well as looking at the genetic causes of deafness it is also important for society to develop ways to improve communication for deaf people so that they live active and fulfilled lives integrated in society."
The findings will be published in the American Journal of Human Genetics in June.