5. Environmental factors you have no control over dictate your testosterone levels. Men's testosterone started declining in the early 70's - around the time plastics were introduced in daily life. The production of plastic and gasoline release vapours in the air which has been decreasing sperm motility for the past 50 years. Each succeeding generation has become weaker than the last but, unless you can somehow stop corporations from creating gas, you are powerless to stop it.