I always thought that some people being able to use the force and some not and that it is something that runs in the family was quite clearly implied in the OT ("The boy is our last hope." "No, there is another." - "Force is strong in my family").
BTW. greenpenguino, the text in your signature is not cool. Frink only cited it from another site because it was so stupid that it was funny and you make it look like it's something he said himself.