There’s the “wrong side” of 30, and then there’s the wrong side of 35. I’ve blown past the former and am quickly creeping up on the latter. In a baseball clubhouse (and, increasingly, in a baseball front office!) I would be the old guy in the room. I can still draw comfort from the fact that there are major leaguers who are older than I am, although more and more, they seem to be guys who carry the tag of “He’s great in the clubhouse!” rather than “He’s great in the WAR column!”

There’s often a lot of talk about teams having “veteran presence” in the clubhouse. There are certain players who seem to have a job waiting for them inside a clubhouse, even though they are “old,” injury-plagued, and not very good any more. But they are still passable major-leaguers and get high marks for being the guys who might rub off on some of the younger players. Perhaps they lead by example. Perhaps they take young players under their wing and help them develop. They’re probably the kind of guys who will show up as managers in a few years.

It makes sense that guys in their mid-to-late thirties would be good candidates to help younger players along. We often think of players in terms of their baseball life cycles, as they go from prospects at 20 to rookies at 24 to their prime at 27 to their decline at 31 and their eventual retirement. But of course, they are also living another life cycle that we rarely talk about. They are … people. At age 35, we hope that a player is only a third of the way through his actual life, but he’s also at an interesting point in his development as a person, especially compared to the guy who is 25.

The psychologist Erik Erikson—who, in one of the great ironies of the field, invented the term “identity crisis”—suggested that humans develop throughout the course of their lifetimes through different stages of life by accomplishing certain critical goals. Early in life, babies are simply trying to establish that they can trust someone in the world. In adolescence, people struggle toward establishing their own identities. In adulthood, they go through a stage where they look to pass along their knowledge to the next generation. Erikson’s model has some flaws. Human development is not nearly so neat and orderly and people don’t always progress through the same order at the same times in their lives. There are some 25-year-olds who are further along the path than are some 30-year-olds. Erikson also thought of the caretaking/passing along knowledge stage as being accomplished by becoming a parent, and of course not everyone becomes a parent, nor wants to be. But often, even people who do not have biological children seek out ways in which they can be mentors to others. They may volunteer with a youth program or perhaps work more indirectly by writing their wisdom out for others. But in the mid-30s, there are a lot of people squarely in what Erikson called the “generativity” stage.

On top of that, something else happens in a person’s mid-30s. The brain finally finishes maturing. We know that, neurologically, the brain is not fully formed until a person reaches their mid-30s. As I am fond of saying, 18-year-olds are “replacement-level” adults, and even 24-year-olds still have a lot of growth left to go. Most of the growth is in the front part of the brain (the pre-frontal cortex) which is responsible for long-term planning, among other things. So we see that 30-somethings are well-positioned as humans to at least be interested (and potentially able) to take on that role of being a mentor to a young player.

But does that actually make a difference on the field? It’s hard to know. There are plenty of anecdotes about players who credit veterans who helped them along the way. It’s hard to dismiss the idea completely out of hand. One area we might look into is the idea of veterans helping young players through the rigors of a long season, or “The Grind.” Veterans have been through a few more seasons. They may know some tricks. We know that over the course of a baseball season batters show a subtle but measurable decline in their ability to avoid strikes. Perhaps teams that have a few veteran guys around have a better chance of beating that grind over time because the veterans are able to get through to the younger guys with lessons on how to take care of themselves and keep going despite being tired.

Warning! Gory Mathematical Details Ahead!
Previously, I looked at the effects of a manager on “The Grind.” The basic method is that we divide all pitches into two categories: those that end in strikes and those that do not. Over the course of a season, even controlling for a hitter’s own abilities, we see a small but meaningful drift toward the bad side. It turns out that managers can have a repeatable and very meaningful effect on their players, i.e., they can save (or not save) a lot of runs.

I used the number of days since Opening Day as my measure of time. I coded all pitches as either ending in a strike or not a strike. (For example, a foul ball with one strike was “bad,” but a two-strike foul was “good.”) For each pitch, I used the batter’s, pitcher’s, and league’s rate of “good decisions” to create a control variable for how likely this particular pitch was to end up as a good decision, using the log-odds method.

I identified how many hitters there were on a given team who were 35 or older when the season started and who had at least 250 plate appearances during the course of the season. That’s not a perfect proxy for veteran presence, but even when I played around with the inclusion criteria (dropped the age requirement to 33 and then 30), it didn’t make much difference.

I created a binary logistic regression with the control variable to start with, and then the number of days since Opening Day, the number of “veterans” on the team, and the interaction of days since Opening Day and veterans. The initiated will recognize this as a standard moderator analysis. The non-initiated are probably wondering what’s going on. There’s a reason I put the #GoryMath warning on this section.

As expected, the “days since opening” variable was significant and pointed toward hitters getting worse over time, but it’s the interaction that we care about. Does having a bunch of veterans bend that line back toward keeping an even keel over the course of a season? We’re already controlling for the fact that some hitters are better than others at controlling the strike zone. Turns out that the answer is no. I looked at the team overall and only the guys 27 and under and it turned out that the number of veterans didn’t have an effect on that slow fading over the course of a season. As I mentioned earlier, I played around a little bit with who counted as a “veteran” and it turned out that the answer was still no.

So, the Veteran Effect Doesn’t Exist?
It might. It might not. My money is still on its existence, but right now, we don’t have evidence for it. I think much of the problem rests in the fact that our regression only knows one piece of data about our veterans, and that’s the fact that they are over 35. The regression doesn’t know if they are the kinds of gentlemen who are vocal leaders or quiet mentors or total sociopaths who just happen to still be amazing at baseball. It’s likely that the 35-year-olds who are still in the game have at least been somewhat selected because they are good clubhouse guys, but maybe we’re asking too much that they would have a team-wide effect. We also don’t know whether the young players around them are the kinds of guys who are far too precious to take advice from anyone else or if they are guys who genuinely would look up to and respect someone who had real knowledge to share.

I think the lesson to be learned here isn’t “there is no veteran effect.” Instead, I’d argue that the simple formulation that a player will be a good clubhouse influence if he is “old” is, well, overly simple. Even if we assume that the players who make it that far are judged by someone to be worth putting into the mix and that those people might have some idea what they’re doing (for the initiated, that we have a biased sample; in fact, teams pay managers and coaches and front-office people specifically to introduce bias into our sample), it’s not as simple as adding an old guy and everything else takes care of itself.

Instead, teams (and we poor saps out here doing public research) need to poke around and find a veteran who is both good at providing some sort of leadership or encouragement or mentorship (and is worth the roster spot). That’s not easy, because to do it numerically involves the kind of data that the public wouldn’t have a chance to see publicly. There are teams that administer personality assessments to potential players, but certainly a lot of it is just talking to people around the league about “what kind of guy” he is. Even then, trying to influence a group dynamic is a very inexact science. There probably is some mathematical equation that governs it all, and the entire field of psychology has only stumbled onto its edges. We know some of the major principles, but sometimes you can have the nicest guy in the world with great interpersonal skills and tons of wisdom to give and he just never gets the chance to do it.

Still, we’ve seen that a manager can have an effect worth multiple wins over the course of a season. If you need to have a 25th man on your roster and you’re going to end up with someone who isn’t going to produce much on the field anyway, it’s worth it for a team to investigate how they might clear some extra value with that spot by finding a guy who might be able to help his teammates fight the grind. It might not be easy, but if a team could crack that code—or even get closer to cracking the code—there’s a place where you can find hidden value.

Thank you for reading

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Why the 250 ab minimum? Maybe the guy you're trying to find is a giambi type who rarely plays. In fact, i might guess that being an old guy who doesn't play much might indicate he's there for his veteran presence.
I thought about that, but I wanted this to be a guy who was around all season, and wasn't just some waiver wire guy that a team picked up on a two-week rental just to patch a hole. I figured there would be more of those than guys like Giambi.
Two things I can think of which might come into play, but might be too complicated to work into the model relative to their value:

Roster days. If a veteran is on the roster a long time, but doesn't get enough at-bats for your cut-off, that's the (Giambi-esque) guy you're looking for, but I don't know how many of them really are out there.

Injuries. Players with minor injuries can often stay around the team, and still have an effect in the dugout and clubhouse. Keeps the at-bats down below cut-offs, but still has sufficient contact time with the kids.
I think the main issue is that you are looking at only one small and probably unlikely potential influence of veteran players. I can think of 4 or 5 things I'd want to look at but "mitigating the grind" was not one of them.
maybe create a variable for the distance from replacement level for the vet and look at the underlying distribution? the reasoning being that the closer the vet is to replacement level, the more likely that he is partially being kept on the roster for his 'intangibles' and not his baseball skills.