Spirits are often bottled at 40% alcohol by volume (ABV), commonly referred to as 80 proof, for a few reasons:

1. Historical Tradition: Along with abolition and religious revival, temperance was a major grassroots movement of the 19th century, In Britain, the motivating factor was the First World War. The government wanted to reduce the alcohol level of drinks so as to minimize drunkenness leading to absence from work that would affect war production. In 1917, after years of negotiations between the government and distillers, they agreed on a 40% maximum level in general, and a maximum of 28.6% ABV in locations critical to the war effort like munitions factories and environs.
2. Regulatory Requirements: In many jurisdictions, 40% ABV is the minimum requirement for a spirit to be labeled and sold as such. This standardization helps ensure consistency and safety in alcohol production and consumption.
3. Flavor and Aroma: At 40% ABV, spirits can express their full range of flavors and aromas while still having enough alcohol content to provide a satisfying kick. This concentration allows for the extraction of various compounds from the base ingredients during the distillation process, contributing to the overall character of the spirit.
4. Mixability: Spirits at 40% ABV are versatile and mix well in cocktails. They provide a good balance between flavor intensity and compatibility with other ingredients commonly used in cocktails.

While 40% ABV is the standard for many spirits, there are exceptions. Some spirits, particularly traditional or artisanal varieties, may be bottled at higher or lower ABV levels based on regional preferences, distiller choices, or specific production techniques.