In both the American League and the National League a starting pitcher must pitch at least five full innings to be eligible for getting credit for a “winning ballgame.” Has this always been the standard? Was it true when Young, Alexander, Johnson, Spahn, etc. were winning their hundreds of games?
Yes. Those guys just pitched a lot more innings. Most of the time the starting pitcher would pitch complete games, and thus always get a decision.
I’m not sure when it was codified, but I doubt the win totals of the pitchers you mentioned would have been affected all that much. Pitchers were expected to pitch nine innnings (especially in the dead ball era), so a game where he pitched less than five and still got a win would be very few. Indeed, if a pitcher were taken out of a game it was either due to injury or the team falling so far behind that it wasn’t worth keeping him in. If the team was ahead, the pitcher stayed in.
It was certainly true when Spahn and Johnson were pitching; it’s an old rule.
It IS theoretically possible for a starter to pitch just four innings and win the game; if the game is called off after five complete innings, with one team ahead, it’s a finished game. In a five-inning game a starter who pitches only four innings can be awarded a win. Rule 10.17(b)(2).
One of the consequences of the development of relief pitching is that if a team is trailing, the manager is more likely to pull a starting pitcher who “just doesn’t have the good stuff” that day. If a team pulls a late-inning rally and wins the game, the starting pitcher won’t get credited with the win.
According to the first edition of the Baseball Encyclopedia,
The effect of all of this on pre-1920 wins is slight. The effect of the five-inning rule is especially slight, because it was all but unheard-of to pull a starting pitcher before five innings when he had a lead. It isn’t terribly common even today.
Any extra win that a pre-1920 starter garnered would be balanced by a win that somebody else didn’t get in relief, and the roles were more interchangeable than they are today. Walter Johnson, for example, made 136 relief appearances in his career.
Caveat and exception…
Immediately following the 1990 lockout a starting pitcher could be credited with a win after completing three innings for the first month of the season due to the truncated spring training that year.