Helle et al. have studied 138 years of human birth sex ratio data, from 1865 to 2003. They find an increased excess of male births during periods of exogenous stress (World War II) and during warm years. In the warmest period over the 138 years, the birth sex ratio peaked at about 1.08 in northern Europe.[29] Increase of sex ratio for 1 °C increase in temperature was approximately the same as the result of Catalano team.[31]