samples. In his book, "The World's Scavengers" (2007), Martin Medina provides a methodological guide to researching waste picking. In 1988, the World Bank estimated that 1–2% of the global population subsists by waste picking. A 2010 study estimates that there are 1.5 million waste pickers in India alone. Brazil, the country that collects the most robust official statistics on waste pickers, estimates that nearly a quarter million of its citizens engage in waste picking. Waste picker incomes vary vastly by location, form of work, and gender. Some waste pickers live in extreme poverty, but many others earn multiple times their country's minimum wage. Recent studies indicate that waste pickers in Belgrade, Serbia, earn approximately US$3 per day, while waste pickers in Cambodia typically earn $1 per day. Official statistics in Brazil indicate that men earn more than women, regardless of age. Approximately two-thirds of Brazil's waste pickers are men overall, but this proportion jumps to 98% in high income waste picker groups (those earning between 3–4 times the minimum wage). No women were found in the highest income groups (those earning over 10 times the minimum wage). Causes In developing countries Over the past half century, in-country migration and increased fertility rates have caused the population of cities in the developing world to mushroom. The global population of urban dwellers is expected to double between 1987 and 2015, with 90% of this growth occurring in developing countries. Much of the new population has settled in urban slums and squatter settlements, which have expanded rapidly with no central planning. The United Nations Habitat Report found that nearly one billion people worldwide live in slums, about a third of the world's urban dwellers. The rapid urbanization greatly increased the demand for informal waste collecting services, as cities lacked the infrastructure and resources to collect the totality of wastes generated by their inhabitants. Despite spending 30–50% of operation budgets on waste management, developing world cities today collect only 50–80% of refuse generated by inhabitants. Residents and businesses often resort to burning garbage or disposing of it in streets, rivers, vacant lots, and open dumps. This is a source of air, land, and water pollution that threatens human health and the environment. Informal waste collectors help mitigate this harm by collecting recyclable materials by foot or in pushcarts, tricycles, donkey carts, horse carts, and pickup trucks. On the supply side, urbanization has facilitated the