If the study is true and accurate, I'd guess the difference in testosterone levels in men from the 50s verses today has to do with wearing sunglasses. Wearing sunglasses and glasses overall was not that popular back then. Instead men wore hats to shield their eyes in the 50s and earlier. Today sunglasses are commonly worn which glass shields the eyes from different light spectrums.
There are some studies finding that bright lights, natural light in particular, can raise testosterone levels a good amount. A little on that and why light works at raising test. levels can be seen here:
Scientists boosted testosterone in men using nothing but a very bright light
http://www.businessinsider.com/scie...-using-nothing-but-a-very-bright-light-2016-9