What Is the Marginal Physical Product of Alcohol?

There have been numerous studies showing that having a drink each day is good for your health. The typical result is that a glass of wine (usually red, but some studies suggest either will do) each day reduces your risk of heart disease.

This article (registration required) reports on a study of women showing that a drink a day, no matter what the form of alcohol, promotes better memory and reasoning as people age.

Those who consumed half a drink to one drink each day for at least four
years were about 20 percent less likely to have an impairment in their thinking abilities and about 15 percent less likely to experience a decline in their mental powers over the two years they were studied, regardless of what type of alcohol they preferred, the researchers found. On average, women who drank moderately tended to have the memory and reasoning agility of someone about a year and a half younger than those who abstained.

Well, if one drink is good, would two be better? What about three? More? At what point do these benefits from drinking level off (i.e. have a marginal physical product of zero) or turn negative?

Unfortunately, the results showed that

No benefit was seen among women who drank more than that [one
drink].

Please note: I titled this posting "marginal physical product" for a reason. I am asking only about the incremental effects of alcohol on brain power. I realize full well that there are other ways in which additional drinks might confer benefits and that a decision as to the optimal amount of drinking would also take into account the expected marginal costs.