Obviously effort is a huge deciding factor on how good you will be, but clearly there is also genetic predispositions that seem silly to say don't play a large part as well. Something about the fact that it's mental, and can't be seen, makes people think it doesn't exist.
Something like 20% of Americans over 7 feet tall played in the NBA. Clearly being over 7 feet tall is an advantage in this area. Proper nutrition plays a huge roll in growth, but not everyone has the genetics to reach this height.
The retort that short people can work really hard to make it is true, look at Allen Iverson (with a relative NBA 'short' of 6 feet tall). He worked really hard to make it, but he worked harder to overcome a disadvantage. What happens when someone works just as hard as Allen, but is 8 inches taller? Any training that Allen does, this other person does as well. It's an uphill battle, that if the taller person keeps pace, they will just win by default.
To say the above is not true, that effort is everything, means you need to explain away why such a high percentage of 7 feet tall people play in the NBA. It would almost imply that for some other, completely unrelated reason, 7 feet tall people just have harder work ethics. Clearly you can have a genetic predisposition to playing basketball, and it's easy to see. Why is it so ridiculous to having a genetic predisposition for formal logic?
What's more, is that I'm sure you would already agree that you can have a genetic predisposition against formal logic. We have people all over the mental retardation spectrum. I realize it's quite crass to talk like this, but there are people with limited mental faculties that through no amount of hard work, would be able to further a field. I don't think you can deny that.
Even if you say disregarding those with a clinical diagnosis for retardation, my counter would then be "Well what about people who just barely sit outside the range of a clinical diagnosis". You would have to expand your definition of who couldn't qualify, and again I would counter with "Well, what about the people who barely sit outside of the range of that criteria?" until it expands so much that nearly everyone is disqualified (including myself).
I would also want to throw out the disclaimer that I'm not saying some people are better than others. We're all 100% human, being human isn't tied to your skills or what you can do. To be better than someone, you would have to be more human than them, which is something that doesn't really make sense. It's not that some people are better, it's that some people are better at certain things.