. . . [F]irms have lower diversification rates as they grow older, and that eventually diversification rates even turn negative for old firms on average. . . . [P]roduct portfolios of larger firms tend to be more diversified. . . . [F]irms experiencing diversification in one period are unlikely to repeat this behavior in the following year.
[The authors] investigate age and diversification patterns of German machine tool manufacturers between 1953-2002 using data concerning product registration in trade journals.
The paper is "Age, diversification and survival in the German machine tool industry, 1953-2002" by Alex Coad and Christina Guenther.