Despite Babe Ruth's dominance in the early years of the lively ball era -- he hit almost 9 percent of ML home runs in 1920, and more than 6 percent in 1927 -- it wasn't until 1931 that the AL began to outslug the NL every year. But there was plenty of slugging to go around, as the peaks and high valleys of 1930-1941 attest. I attribute the higher home-run output of those years to arrival of a new generation of players, who were selected more often than not for their slugging ability and encouraged to cultivate that ability.
But the real lively ball eras were yet to come:
Following a lull from 1942 through 1946, the home-run barrage resumed in 1947, with the post-war return of slugging veterans and the influx of newcomers raised in the slugging tradition. The second lively ball era peaked in 1961. It subsided with the "era of the pitcher" and the first waves of expansion. But even at its lowest ebb in the 1970s and 1989s, the pace of home-run production exceeded the peaks of the first lively ball era, with only a few exceptions.
Then came 1994 and a third era. This one, sad to say, probably owed its existence not to a "juiced" baseball but to "juiced" baseball players. Given the crackdown on performance-enhancing substances, the rate of home-run production in 2010 (to date) has dropped to that of 1961 -- when the "juice" in the game came from a performance-inhibiting substance known as alcohol.
I hereby declare the following eras:
1901-1919 -- Deadball ("modern")
1920-1941 -- Lively Ball I
1942-1946 -- Wartime Lull
1947-1961 -- Lively Ball II
1962-1993 -- High Plateau
1994-2xxx -- Juiced Player