The following is from p. 449-450 of The New Bill James Historical Baseball Abstract:

Lee May had a younger brother, Carlos, who also played in the majors. Carlos got 80% of the talent in the family; he was as strong as Lee, which was very strong indeed, but he could also run, and he could throw a little bit. He was a natural hitter, a lefty, and he knew the strike zone. when he was 21 years old, he looked like the second coming of willie Mays.

Carlos blew his thumb off in a military training accident, which cost him a lot, but there was another problem. Whereas Lee May was a hard-working, hustling player within the limits of his ability, a respected clubhouse leader in the early days of the Big Red Machine, Carlos played baseball as if he didn’t really give a shit, pardon my French. I have noticed this same syndrome other times, that when you have brothers who are baseball players, very often the youngest brother has or is credited with having the most talent in the family, but turns out to be the one who does the least with it.

…

As the Alous brothers were emerging, Jesus Alou was the one who was always thought to have real, big-time talent, but he never did very much with it. I think that the reason this happens, perhaps, is that when a boy has several older brothers who play ball, he may play a lot of baseball at a very early age. Because the kid has played way more baseball than the other kids his age, he may be years ahead of them in the development of his skills—and it will look, to those around him, like he has just world of talent.

But in reality, the younger sibling is being pulled along by his brothers’ interest in the game, while his own interests and his own focus, once it develops, may be somewhere else. Of course, once in a while you get the younger brother who does have the focus and the desire to excel, and then you get Joe DiMaggio or George Brett. But there aren’t very many Joe DiMaggios or George Bretts. Most of the time, you get Hector Cruz or Jesus Martinez.

I would love to add my own piercing personal insight to those thoughts, but I have no brother, and don’t always understand the nuances of that relationship. People often say having a true best friend is akin to having a brother, and in one way they’re right, but in another way, that’s stupid and not at all useful, and this situation is more like the second way.

Justin Upton and B.J. Upton are teammates on the Atlanta Braves right now. One of them arrived as a free agent, one whose effort and intensity has been questioned by teammates in the past. One arrived by trade, after basically two and a half years of his team’s ownership, front office and coaching staff intimating without much evidence, but also without much equivocation, that he is lazy, unfocused, mentally fragile, physically fragile, or some mixture of them all. Both are wildly talented, and both have been superstars for full seasons (B.J. in 2007-08, Justin in 2009 and 2011). Both have also had solid, but unspectacular seasons (B.J. 2009-12, Justin 2010 and 2012). Both played more than most people play before their 22nd birthdays, but neither was especially good before their 22nd birthdays. Neither has been less than a solid-average player for a full season since then.

Yet, both are treated as having imperfect makeup; both are treated as if they have tremendous potential into which they have not yet tapped. Both are considered, if not disappointing, at least enigmatic.

Justin certainly has gotten the worst of it all, mostly because he played for so long in the shadow of bizarre trade rumors. He plays right field, so it’s more alarming when he doesn’t hit well. He is younger, so he has less of a track record of success. He has much more prodigious power, which always lends itself to overstatement of a player’s overall potential. He had the best single season either player has ever had in 2011, raising expectations to a different level.

I don’t sense that Justin feels pulled along, or that he does not, as James wrote of Carlos May, “give a shit, pardon my French.” To the contrary, he seems to have fun playing the game. He watches his home runs, but not in an insufferable way, just in a loving way. He hustles on the bases, hustles in the field, seems very thoughtful in interviews in terms of how he approaches the game. I don’t think he resents or dislikes baseball.

I will say, though, that something seems to be missing for the younger Upton. He seems almost too unfazed by outside problems. An ESPN: The Magazine feature on him from this summer left me with the impression that the man could not care less what the media, fans or even his ownership thinks of him. That’s great for him as a person, but a little bit of a problem for him as a player.

The optimal psychological frame for any athlete will be slightly different from the same for all others, but there are a few things I have come to believe are nearly universally true in this area:

1. Baseball requires a certain obsession. Single-mindedness makes you great, or at least allows you to tap fully into your potential. 
2. An especially centered, settled life can leave one too willing to allow the comforts of home to break tension created during a hard day at the park.
3. Without external pressure and scrutiny from which to try hard to get away, one rarely has the necessary impetus to really get inside oneself, drill on swing mechanics, be willing to stay out of the clubhouse for an extra hour to work on something.

I think Justin will be much better off in Atlanta. When he said of playing with B.J., “we’re going to butt heads a little bit. That’s going to be the fun part,” I took notice. That’s exactly the idea. let your teammates push you, let your family push you, and allow the external stuff to keep you going a few minutes longer, dwelling a little bit deeper in the game, than you would otherwise.

I think Justin is likely to be happier in Atlanta, and that’s great. I’m not advocating horrible home environments, excessive drinking or the like just so players can perform better. With more motivation and maybe a touch of testiness, Justin might be able to shake the little-brother syndrome and reach his full potential in Atlanta.