Caín AltoCaín Alto is a barrio in the municipality of San Germán, Puerto Rico. Its population in 2010 was 2,830.HistoryCaín Alto was in Spain's gazetteers until Puerto Rico was ceded by Spain in the aftermath of the Spanish–American War under the terms of the Treaty of Paris of 1898 and became an unincorporated territory of the United States. In 1899, the United States Department of War conducted a census of Puerto Rico finding that the population of Caín Alto barrio was 1,643.