Between 2005 and 2015, India, along with Ecuador, posted the biggest increase among origin countries for eligible immigrants opting to become American citizens, a study has found. By 2015, as many as 80 per cent of eligible immigrants from India opted to become American citizens, as against 69 per cent in 2005, thus registering an increase of 12 percentage point, the Pew Research Center has said. Ecuador from Africa also registered a similar 12 percentage point increase during the same period. During this period, the total number of naturalised immigrants in the US increased from 14.4 million in 2005 to 19.8 million in 2015, a 37 per cent increase. "By 2015, eligible immigrants from India had one of the higher naturalisation rates (80 per cent) due to a 12-percentage-point increase in its naturalisation rate since 2005. Only eligible immigrants from Ecuador (68 per cent in 2015) had as large an increase," Pew said. "This is a bigger increase than for US immigrants overall, among whom naturalisation rates jumped from 62 percent in 2005 to 67 percent in 2015," the research center said, adding that eligible immigrants from Vietnam (86 per cent) and Iran (85 per cent) had the highest naturalisation rates of any group in 2015. However, the naturalisation rates among eligible immigrants from Honduras, China and Cuba declined or remained largely unchanged from 2005 to 2015, as per the most recent year for which Pew Research Center estimates are available. To be eligible for US citizenship, immigrant must be at least 18 years old, have lived in the country for at least five years as lawful permanent resident or three years for those married to a US citizenship and be in a good standing with the law. Between 2005 and 2015, the United States denied naturalisation applications to nearly one million immigrants.