One of the more interesting debates going on in baseball today is the discussion of whether plate discipline is an innate tool or rather a skill that can be honed and refined. Personally, I have always come down on the side of plate discipline being a tool. People always talk about the 5 tools (power, batting average, speed, defense, throwing arm), but in actuality it should be 6 tools (Those 5 + plate discipline).
Major League Baseball players have been playing for years and years before they ever reach the show. And, in all that time, they are using a consistent approach at the plate. It seems from the moment they pick up a bat, they are the kind of hitter that they will always be. Some are patient, while others are aggressive. Just like some players being fast or having cannons for arms, while others have lead feet and poor arms. While a player may be able to make an incremental improvement in these tools through better mechanics or conditioning, by and large raw tools can't be improved very much.
However, others do believe that plate discipline can be taught, that with the proper instruction a hitter can change his approach. Personally, I'd say that that would be the exception, not the rule. Even so, the most widely cited case for improved plate discipline is New York Met shortstop Jose Reyes, who has reputedly made incremental improvement since his rookie year. Even so, I'm not sure if it's legitimate improvement or just the result of a rookie year that wasn't indicative of his true plate discipline. His career minor league batting average and OBP are .285/.338, while his career MLB numbers are .287/.336. So, his current level of performance is exactly in line with what he did in the minor leagues.
Anyway, I thought it might be fun and maybe even instructive to take a look at some of the brothers who have played the game. Maybe that will reveal something about the nature vs. nurture debate. Granted, a lot of these players had similar upbringings, but given that they have played for different coaches and different organizations, I think it's fairly clear that plate discipline is much more likely to be a tool than a learned skill.
Here are the IsoD (OBP - BA), Plate Appearances per Walk, and percentage of Plate Appearances that result in a Walk for some of the brothers who have played professional baseball:
I find the similarity between the plate discipline skills of these sets of brothers to be pretty remarkable. The Boones are almost exactly the same, as are the minor league numbers of the Ka'aihues, and the MLB numbers of the Giambis. The DiMaggios were all pretty close, though Dom had a leg up in controlling the strikezone. The Alous were all free swingers, but Jesus was much less patient than Felipe and Matty. Billy and Cal aren't too far off, but it's clear that Cal was more patient. Clearly, the biggest discrepancy is between Vlad/Wilton and Paul/Lloyd. In both cases, one of the brothers (Vlad and Paul) has significantly more power than the other (Wilton and Lloyd), so it's possible that pitchers worked more carefully to those hitters and give them more "unintentional, intentional walks."
Overall, I think genetics do determine the type of hitter you are. You can argue that the fathers taught the kids the same approach at the plate, but I think there's more to it than that, especially considering just how many different hitting coaches each of the brothers have had in their careers. That said, what we really need to resolve this pesky "nature vs. nurture" debate is a set of twin MLB brothers who were separated at birth and raised in completely different environments, but until that time I suppose we'll just have to settle for this type of anecdotal evidence.