https://www.mcrmfertility.com/wp-content/uploads/2021/05/Glo... mentions "No clear etiologic factors have been identified; however, multiple factors have been shown to have correlative associations" then lists some possibilities:

* Increased obesity.

* Changes in diet, to have more fats.

* More people with chronic diseases like diabetes and hypertension.

* Environmental toxins including bisphenol A (BPA), phthalates, triclosan, and air pollutants.

* Tobacco, including "parental tobacco exposure in utero has been reported to affect male fertility in the offspring."

* Increased marijuana use ("though the field is under-researched")

* "Sedentary lifestyle, stress, and lack of sleep"