Men's testosterone levels have declined significantly over recent decades — studies show roughly a 1% annual drop, which adds up to about 10–20% over a decade. Research covering tens of thousands of men across multiple countries documents this consistent downward trend. Scientists like Hagai Levine have highlighted this decline as a concerning pattern in male reproductive health, though the research documents the hormone trend itself rather than a proven population-level fertility crisis. The decline appeared even in men with normal body weight, suggesting it's not simply driven by rising obesity. What remains unclear is whether this reflects environmental, dietary, behavioral, or biological factors — the studies document the trend but pinpointing causes requires further investigation.