Humans have a very high sensitivity to bitter tastes, which comes from our need to recognize potential toxins. We can even distinguish 300 different kinds of bitterness! Because our threshold for detection is so low, bitters are typically added to cocktails in dashes rather than ounces. But even in this low concentration, the effect can be dramatic. Try making a Manhattan or Old Fashioned with and without bitters for comparison.
Bitterness has the effect of slightly suppressing other tastes (sweetness, sourness, and saltiness), which creates a rounder, more balanced final result. The term “cocktail” originally meant a drink that combined spirits, sugar, water, and bitters. Before appearing in cocktails, bitters were consumed medicinally – adding them to alcohol just made them easier to get down! Even today, a few dashes of an aromatic bitters such as Angostura or Peychaud’s in a glass of club soda can be a potent stomach-settler.