Every season, ballplayers in the last year of their contracts play horribly. This becomes the cause of much fretting and concern about the impact of money on a player’s game. But this seems reasonable: Someone playing for a new contract is under a lot of pressure, and those under pressure can sometimes react badly to it.

Oddly enough, every year, ballplayers in the last year of their contracts also play wonderfully, and this too occasions much fretting over the effect that money has on their games. Fans and writers wonder whether these players are working harder with the idea of making more money, a prospect that leads to unpleasant notions about how hard they’re working when cash isn’t on the line. This, too, seems reasonable. Baseball is too difficult a game to imagine that anyone could simply become good at will so as to earn more money, but people under pressure sometimes react well to it, and the promise of tens of millions of dollars can serve as a powerful painkiller and an incentive to concentrate.

How to explain all this Jekyllism and Hydery? Does money really weigh on athletes this much?

Although the salary drive is clearly one of the reasons affecting whether those playing on expiring contracts play well or poorly, there are other, more important factors at work. The most important is that nearly all free agents are old and good. They’re all old because a player has to play at least six years in the major leagues before he can become a free agent. With the odd exception, a player is usually nearing the end or is past his prime when he hits the market. And free agents are all talented because you have to be to play six years in the major leagues.

As obvious as this is, it’s something one has really to consider when examining the success of the current free agent class. You can’t look at a group of old, good players and expect them to perform the same way a broader group of players would. The older crop is more liable to play badly, relative to its own standards, than a random group of players, simply because they’re old; they’re also more liable to play well, relative to their own standards, because they’re good. This, more than the effects of contracts weighing on their minds, probably better explains why pending free agents seem to either have career years or catastrophically bad ones.

Consider, for instance, several prominent center fielders who will be hitting the market this winter, among them Ichiro Suzuki, Andruw Jones, Torii Hunter, Mike Cameron, and Aaron Rowand. The oldest, Cameron, is 34; the youngest, Rowand, turns 30 in August. All of them have been excellent, even dominant, defensive players throughout their careers, and all have been very good, and occasionally, great, hitters. Because they’re a relatively old group, you’d expect some of them to be having bad seasons this year, and because they’re a group of very good players, you’d expect a number of them to be having banner years.

As it turns out, the distribution of performance among the five players is very neat, and would fit a bell curve nicely. Hunter, who’s batting .317 BA/.350 OBA/.581 SLG, is having the quintessential contract drive, playing far better than he ever has before. So too is Rowand, who’s hitting .325/.392/.503. Ichiro, at .335/.391/.452, better than he has for the last two years. Meanwhile, neither Jones nor Cameron is hitting above .225 or making up for their poor batting average in other areas.

A look at other positions exposes fairly similar patterns. For instance, some of the top pitchers, such as Livan Hernandez, who’s putting up a 3.58 ERA in Arizona’s fine hitter’s park, are pitching far better than anticipated, while others, such as Carlos Zambrano, who has yet to knock his ERA below 5.00, are pitching much worse. Still, overall, as a class, they’re pitching about as well as you’d expect.

It isn’t just positions that show this pattern; it shows up on teams, too. Look at the Yankees and their many potential free agents. In the Bronx, fans are bemoaning Mariano Rivera’s terrible start and many blown saves, and mourning the death of Bobby Abreu’s bat; more than a few have speculated that Rivera’s contract situation, in particular, might be hurting the team. On the other hand, Andy Pettitte is pitching like a serious Cy Young contender; Jorge Posada, of all people, is in the running for the batting title, and Alex Rodriguez is having the best season of a magnificent career. It’s about what you’d expect from a lot of old, good players: Some wild underperformance, some wild overperformance, and on the whole, about the level of performance you’d have expected coming into the season.

Players aren’t automatons or weighted dice, but looking at the free agent class this way points up the predictability of the unpredictable performance of ballplayers looking to cash in. Barring specific information regarding injuries, mental frailty, and other such factors, a pending free agent enjoying a monster year or a dismal one shouldn’t really be regarded any different from any other player. You might consider his recent play more heavily than how he’s played in the past, but not that much more heavily. Hunter is not really an MVP-caliber hitter, and Zambrano is not a 5.24 ERA pitcher. They’re just players doing more or less what could be expected of them when they’re judged as part of a larger group. Neither is going to suddenly become something other than what they are just because of contract concerns. Anxiety and concern might be best left to players’ agents. They get paid to worry.

tmarchman@nysun.com