Effect of wishes on character ability scores 1979 AD&D DMG

It is quite usual for players to use wishes (or alter reality spells found on scrolls) to increase their ability scores in desired areas, whatever the areas might be. It is strongly suggested that you place no restrictions upon such use of wishes. However, at some point is must be made more difficult to go up in ability, or else many characters will eventually be running around with several 18's (or even higher!). Therefore, when any ability score reaches 16, then it should be ruled a wish will have the effect of increasing the ability by only 1/10th of a point. Thus, by means of wishes (or wishes and/or alter reality spells) a charisma score of 16 can only be raised to 17 by using 10 wishes, the score going from 16 to 16.1 with the first wish, 16.2 with the second, and so on. This is not to say that magical books or devices cannot raise scores of 16 or better a full point. The prohibition is only on wishes.