Household size — the number of people living together under one roof — has been shrinking worldwide, and the trend could have major consequences for resource consumption, new research finds. Although global population growth has been somewhat curbed in the developed world, the number of households has continued to grow at a much faster pace in nearly all countries, Michigan State University researchers found. Average household size in developed nations declined from approximately five members in 1893 to 2.5 in 2000, while the rapid decline in household size in developing nations began around 1987, according to the research, which analyzed trends between the years 1600 and 2000. Smaller households are typically less efficient, with fewer people using proportionally more land, water, and energy. Constructing housing units also consumes lumber and building supplies, and generally requires building more roads and commercial areas. “This will put enormous strain on the environmental life support system we rely on, even if we achieve a state of zero population growth,” one study author said.