A certain city has had many vacant homes in the downtown neighborhoods. The city loses significant tax revenue each year because those houses stand unoccupied. The city felt that the fear of crime was keeping professionals from moving into these houses. Over 80% of the city's professional work force live outside of the city, in nearby suburbs, and commute daily.
Plan: Five years ago, the city tripled the size of the municipal police force, increasing patrols throughout the city. All studies have confirmed that crime has dropped substantially.
Result: the vacancy rate in these single-family houses has not changed substantially in five years.
Further information: Almost all of the vacant houses are in excellent condition, ready for occupancy. The city supports a number of high-paying professional jobs, and the state economy has been booming over the past five years.
In light of the further information, which of the following, if true, does most to explain the result that followed the implementation of the plan?