Speculation a During 2006, 22%of homes purchased (1.65 million units) were for investment purposes, with an additional 14%(1.07 million units) purchased as vacation homes D Economist robert shiller argues that speculative bubbles are fueled by contagious optimism, seemingly when prices are rising.. often takes hold impervious to facts, thatS li pecu ation  During 2006, 22% of homes purchased (1.65 million units) were for investment purposes, with an additional 14% (1.07 million units) purchased as vacation homes.  Economist Robert Shiller argues that speculative bubbles are fueled by "contag g ious optimism, seemingly impervious to facts, that often takes hold when prices are rising.