The study, led by Israeli epidemiologist Hagai Levine, updates 2017 research which had come under scrutiny for only including North America, Europe, Australia and New Zealand."Furthermore, data suggest that this worldwide decline is continuing in the 21st century at an accelerated pace," said the study published in the journal Human Reproduction Update.'We genuinely don't know why'Sperm count is not the only factor that affects fertility -- the speed of sperm movement, which was not measured in the study, also plays a crucial role."Exposure to pollution, plastics, smoking, drugs and prescribed medication, as well as lifestyle, such as obesity and poor diet, have all been suggested to be contributory factors although effects are poorly understood and ill-defined."While hailing the "very elegant meta-analysis", Pacey said he believed we have "simply gotten better" at the difficult task of counting sperm, which could account for the falling rates."