10 The Big Bang Theory Fan Theories That Will Blow Your Mind
9. Sheldon And Leonard Were Both Adopted
The more you think of Sheldon and Leonard's relationships with their mothers, the more this theory makes perfect sense.
Sheldon's mother, Mary, is the exact opposite of him. She doesn't pour tedious facts down your throat, and she's able to adapt herself to social situations in order to not upset anyone. Similarly, Leonard's mother, Beverly, is the polar opposite of him. The Hofstadter matriarch may also work in the science world, but she won't back down from leaving her peers upset, even going so far as to question what moment(s) in their life led them to where they currently are (for example, Penny and her many, many terrible relationship choices that led her to Leonard).
The main point here is that both Sheldon and Leonard don't compare to their mothers in many ways, if any. It's because of this that fans have speculated as to whether they could be adopted, particularly in the case of Leonard.
It suggests that Leonard was adopted by Beverly and Alfred for the sole reason that it means they could raise a child to study his development from child to man. Knowing Beverly, it wouldn't exactly be an outside the box move for her.