I haven't read any studies and can't really point you in the direction of one.... but I have an opinion 
I think our geckos' personalities are from both nature and nurture. I think the more tame your leo is - it's more of nurture. If leos were able to run wild and free or have never been really hand tamed before, they'll act how they do in nature - which is avoid contact. They'll hide under something dark, hiss and stand on all fours with their tail swirling, or drop their tail before they make an escape for it. When leos are babies, they tend to be more jumpy and jittery because they don't know what to expect with this huge hand coming into their territory. But, as they grow accustomed to it being there, feeding them, and letting them walk around more places than just their home, the less likely they are to hiss at you, run from you, or drop their tail when they see your hand again in the future.
The first 1 or 2 weeks I had Melvin, he did not want to crawl on me or be picked up unless he was standing on his hind legs and leaning against the glass. When he saw my hand in his tank, he'd look at it and walk away and go in his cool hide. He'd still come out to eat, but didn't really want to make contact with me. But now, Melvin got so used to my hand being there to pick him up and let him crawl on me - that he tries to crawl up my hand while I'm feeding him and putting food in his bowl
lol - he'll run to the bowl, stand on the handle (looks like a swirled branch) of the dish like he always does, and keep his arms somewhat elevated off the dish with his head looking up. The closer I bring my hand in to put a couple mealies in his dish, he raises his head higher and licks my fingers and tries to climb. Because of this, I end up holding him for 10 - 15 minutes at a time before feeding time, put him back in the tank on the branch of his food dish, and he goes off to attack the little mealies and then goes to rest on top of his humid hide or wander about trying to climb up the glass with many unsuccessful attempts. It's really cute and now we have our own little schedule. Once it's night time and I head over to the tank, he pokes his head out and makes his way out - ready to be handled. This schedule started happening 2 weeks after I got Melvin (started beginning of last week and still happening!
)
But that's only my opinion, so you don't have to take my word for it
But see - your leos wouldn't be the same if you didn't hand tame them and keep them in captivity right? 
aimee
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1.0 Albino Leopard Gecko (Melvin)
1.0 Black Lab / Pitbull mix (Mikey)
1.0 Blue Fighting Fish (Blue Balls)