Korea’s ratio of male to female births hit the lowest level since data first began to be collected in 1981, according to Statistics Korea on Tuesday. The state agency said there were 105.3 male births per 100 female births last year.

The agency said the reasons behind Korea’s lfalling male-female birth ratio is the decreasing number of people who prefer boys to girls.

Sex ratio at birth is the number of male births per 100 female births, and a number between 103 and 107 is considered normal.