Using international data for 18 vintages, we find systematic differences between the actual and stated alcohol content of wine. Our results suggest that rising alcohol content of wine may be a nuisance by-product of producer responses to evolving market and production environments.
See article here: Vinum Verum Viribus? Systematic Errors in Wine Alcohol Labels by Julian M. Alston, Kate B. Fuller, James T. Lapsley, George Soleas, and Kabir P. Tumber from ARE Update Vol. 19, No. 4, Mar/Apr, 2016