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A small village in India is seeing an explosion in the number of twins born 鈥� and no one knows why.

Kodinhi, in the southern state of Kerala, now has one thousand twins in a population of eleven thousand and the twin birth rate is still increasing. 鈥淚t鈥檚 out of this world,鈥� says Lorena Madrigal of the University of South Florida.

The background rate of twin births in India is about one in a hundred, so the twin numbers in Kodinhi stand out.

The phenomenon came to global attention in 2009, but the explanation is a mystery. So Madrigal鈥檚 team went to the village to see if they could find any clues about possible genetic or environmental causes.

They spoke to households that included nearly half the twins in the village, and drew up family trees comprising about 1800 people going back to the 1860s. Though they didn鈥檛 manage to pinpoint why the trend was happening, they did manage to rule out a few suggested explanations.

Firstly, marriage within families does not to appear to be the cause. As some genes raise the chance of having twins, it was thought the village鈥檚 tradition of marriage between first cousins and between uncles and nieces could be concentrating these genes in certain families. However, the team found twins were not more likely from such unions.

The recent start and steep rise in twin births also points against cousin marriage being the trigger, says Madrigal. 鈥淭hey have always practised it.鈥�

Another possible explanation suggested in previous media reports was a past disease epidemic. People with genes that favoured twins could have been more likely to survive the disease for some reason. But the older villagers interviewed recalled no such epidemic.

Additionally, there was no link between twins and where in the town people lived.

The only previous published research on the village also managed to rule out as the cause.

Madrigal and her colleagues weren鈥檛 able to take blood samples to see if the twins are identical or not. It鈥檚 impossible to tell by their looks, as fraternal twins can be very similar, says Madrigal.

Twins peak

The team found twin births in Kodinhi began to rise in about 1960, and have been increasing ever since, although the rate of increase may now be slowing down.

In twin-prone families, twin births overtook singleton births around 1990 and now there are three times as many twin births as singletons.

There have also been three sets of triplets born, although not all survived. 聽The background rate of triplet births is about one per 10,000 in India, so this is also higher than normal.

Although she has no proof, Madrigal thinks the most likely explanation is that some residents were once exposed to something that triggered chemical changes to their DNA, which is now being passed on to their children. Her team hope to return to get blood samples in future to shed light on this question.

There have been about six other clusters of twin births in various places around the world, although none have shown as steep an increase as this, says Joseph Bonner at the University of Arkansas for Medical Sciences, who is now working with Madrigal.

Twins are more likely to be born prematurely and underweight, so one factor that could be contributing to the increase is that twins would have been more likely to die in the past, due to less medical care, says Tim Spector at King鈥檚 College London. 鈥淚t might be that they鈥檙e more likely to live now.鈥� This could be amplifying some tendency to have twins, but it couldn鈥檛 be the whole story.

The work was presented at the recent American Association of Physical Anthropologists meeting in Cleveland, Ohio.