I can see where everyone is coming from and thats certainly a side of the coin that I can see. On the other hand there are several personality tests that are suggested for people to take to determine what job they should do/are most adapted to. Which makes me think that if CEOs of fortune 500 companies swear by using these tests than maybe its of use for horses.
Personality is a factor of environment just like people; however, I don't think its entirely dependent on the handler. There are horses that spook and act like dinks even in the most capable hands. Likewise, there are horses that are total sweet hearts and never set a toe out of line even though their people give them 1001 reasons to. Which creates a nature/nurture debate.
I think an important factor here is the amount of horses that are actually spooky, as in, it's just their personality... Like ButtintheDirt said, a lot of times, people will say that it's their horse's personality that makes them constantly all over them, when in fact it's just their 'training'- they get 238,3945 treats twice a day. Misidentification of personality is pretty common; very few people seem to be able to discern nature from nurture.
So far, I've only met one horse that was just plain timid, he was at the bottom of the totem pole in our herd and he required
a firm rider with tons of confidence, because no matter how much training he had, no matter how much experience he got, he needed the reassurance of someone else. It was his personality. But the amount of horses like him that I've encountered has been infinitesimal. The majority of the time, a 'spooky' horse just needs more exposure/ training/ experience; it's not actually their personality.
A good example is the arabian breed. They're known for being flighty and spooky, but if you ask people actually familiar with the breed, most of them will tell you that they have never met, or very rarely met, an arabian true to that stereotype. And of the ones that were spooky and flighty, how many were just mishandled, and needed more training? The only arabian I know that fits the spooky/flighty stereotype is one that has holes in his training as big as Texas.
Horses that are raised and trained correctly, the vast majority of the time, will be friendly, calm horses. Few are like that from that get-go because it's their personality, some need the training to cement and enhance the aspects of their personality which make them that way, etc.
I'll stand by this with personality tests for people, especially- the thing is, people lie, intentionally or not. How can you know for sure that that person even knows what their real personality is, and what is just 'nurture'? They could be answering with what they want their personality to be, and, like I said, not many people can differentiate between what's nature and what's nurture, and that can make a huge difference.