Secularists invented effective birth control methods could now be in danger of dying out because religious people are having more children, a new study is claiming.

Traditional religious followers tend to have more children than atheists with the scientists suggesting this will lead to the decline of secularism across the world in the 21st century. 

The American and Malaysian researchers asked more than 4,000 students about their religious beliefs and number of siblings.

They found in Malaysia, atheists had on average 1.5 fewer siblings than the average.

That gap was narrower in the US but people affiliated with a religious group still had on average more children than those unaffiliated.

Although birth rate differed across religious groups with Muslims being the most fertile and Jews and Buddhists being the least, the trend was noticable, they study claimed.

'Secularism is likely to undergo a decline throughout the remainder of the twenty-first century, including Europe and other industrial societies,' they conclude.

'It is ironical that effective birth control methods were developed primarily by secularists, and that these methods are serving to slowly diminish the proportional representation of secularists in forthcoming generations,' the researchers say.

They claimed the change has mostly come about in the last century and before that birth rates were largely the same across religious beliefs.

'By the mid-19th century, scientific discoveries had moved to a point that human reproduction was sufficiently well understood that fertility rates began to be impacted, especially in the emerging industrial countries,' the scientists say.

'Research indicates that the individuals who were most successful in curtailing their fertility during this time were the most highly educated and the least religious,' they add.

'Thus, for the first time in human history, secularists began to curtail their reproduction much more than the highly religious segments of these countries.'