Sorry that wasn't what I meant. Not physical strength, not mental strength, but Force strength. AKA midi-chlorian count.
Midi-Chlorian were in all life forms. I don't believe that was the reason someone chose the dark side over the light side. I still believe choosing dark over light comes down to a personal choice . Midi-Chlorians had no bearing on that.
Take for example Darth Plagueis he tried to manipulative the midi-chlorians.
Read this from (http://starwars.wikia.com/wiki/Midi-chlorian
In the decades before the Battle of Naboo, Dark Lord of the Sith Darth Plagueis commenced his own experiments with the midi-chlorians. While the Jedi thought of the cellular organelles as symbionts, Plagueis believed they were interlopers, running interference for the Force, and standing in the way of a beings' ability to contact the Force directly. Through years of experimentation, Plagueis had honed an ability to perceive the actions of midi-chlorians. Furthermore, Plagueis believed that only the Sith understood that sentient life was on the verge of a transformative leap; that through the manipulation of midi-chlorians, or the overthrow of the Forceful group that supervised them, the divide between organic life and the Force could be bridged, and death could be erased from the continuum. In order to further the Grand Plan, Darth Plagueis, and his apprentice, Darth Sidious, committed an act in direct violation to the nature of the Force, and attempted to tip its balance toward the dark side. The experiment yielded unfruitful however, and the midi-chlorians, unwilling to comply, struck back in retaliation conceiving a savior to ultimately destroy the Sith: the prophesied Chosen One, Anakin Skywalker