Provides an investigation of the time patterns of ratification for four core ILO Conventions, the Right to Organise and Collective Bargaining Convention, 1949 (No. 98), the Abolition of Forced Labour Convention, 1957 (No. 105), the Discrimination (Employment and Occupation) Convention, 1958 (No. 111), the Minimum Age Convention, 1973 (No. 138), using ratification and other data for 97 countries from 1950-92. Estimates the probability of ratification for a country in any year, given that it has not so far ratified the Convention. Finds that basic economic variables, real income per capita, degree of openness to trade, education levels, degree of urbanization do not explain the probability of ratification. Neither does the political rights variable, which has been suggested by some theorists.