Occasionally, a housing black market develops, wherein landlords withdraw houses and apartments from the market, making them available only upon payment of additional key money, fees, or bribes—thus undermining the rent control law. Many such laws allow "vacancy decontrol", releasing a dwelling from rent control upon the tenant's leaving—resulting in steady losses of rent-controlled housing, ultimately rendering rent control laws ineffective in communities with a high rate of resident turnover. In other cases social housing owned by local authorities may be sold to tenants and then sold on. Vacancy decontrol encourages landlords to find ways of shortening their residents' tenure, most aggressively through landlord harassment. To strengthen the rent control laws of New York, housing advocates active in rent control in New York are attempting to repeal the vacancy decontrol clauses of rent control laws. The state of Massachusetts abolished rent control in 1994; afterwards, rents rose, accelerating the pace of Boston's gentrification; however, the laws protected few apartments, and confounding factors, such as a strong economy, had already been raising housing and rental prices.