Height-for-age standards

The World Health Organization (WHO) has produced a comprehensive set of growth stan­dards for children. These standards are based on studies of children living in 6 nations on 5 continents under optimal conditions with respect to health and nutrition. The table dis­plays the percentile distribution of height, in centimeters, at 3-month intervals, for boys ages 2 through 5 according to the WHO model. In a model population—a large population of boys ages 2 through 5 that conforms to the WHO growth standards—for n = 3.15, 50. 85. and 97, the nth percentile in height for a given age is the unique height among boys of that age that is greater than or equal to n percent, and less than or equal to (100 - n) percent, of heights of boys of that age.

Weight-for-height standards

The graph shows the percentile distribution of weight, in kilograms, for heights from 80 cm to 120 cm. for boys ages 2 through 5, according to the WHO model. In a model population, for n = 3,15,50,85, and 97. the nth percentile in weight for a given height is the unique weight among boys of that height that is greater than or equal to n percent, and less than or equal to (100 - n) percent, of weights of boys of that age.

    Scientists generally credit violent collisions between tectonic plates, the mobile fragments of Earth's rocky outer shell, with sculpting the planet's surface, as, for example, when what is now the Indian subcontinent collided with Asia, producing the Himalayan Mountains. However, plate tectonics cannot fully explain certain massive surface features, such as the "superswell" of southern Africa, a vast plateau over 1,000 miles across and nearly a mile high. Geologic evidence shows that southern African has been slowly rising for the past 100 million years, yet it has not experienced a tectonic collision for nearly 400 million years.The explanation may be in Earth's mantle, the layer of rock underlying the tectonic plates and extending down over 1,800 miles to the outer edge of Earth's iron core.

    Since the early twentieth century, geophysicists have understood that the mantle churns and roils like a thick soup. The relative low density of the hottest rock makes that material buoyant, so it slowly ascends, while cooler, denser rock sinks until heat escaping the molten core warms it enough to make it rise again. While this process of convection was known to enable the horizontal movement of tectonic plates, until recently geophysicists were skeptical of its ability to lift or lower the planet's surface vertically. However, recent technological advances have allowed geophysicists to make three-dimensional "snapshots" of the mantle by measuring vibrations, or seismic waves, set in motion by earthquakes originating in the planet's outer shell and recording the time it takes for them to travel from an earthquake's epicenter to a particular recording station at the surface. Because geophysicists know that seismic waves become sluggish in hot, low-density rock, and speed up in colder, denser regions, they can now infer the temperatures and densities in a given segment of the interior. By compiling a map of seismic velocities from thousands of earthquakes across the globe, they can also begin to map temperatures and densities throughout the mantle. These methods have revealed some unexpectedly immense formations in the deepest parts of the mantle; the largest of these is a buoyant mass of hot rock directly below Africa's southern tip. Dis-pelling researchers' initial doubts, computer models have confirmed that this formation is buoyant enough to rise slowly within the mantle and strong enough to push Africa upward as it rises.

    Many politicians, business leaders, and scholars discount the role of public policy and emphasize the role of the labor market when explaining employers' maternity-leave policies, arguing that prior to the passage of the Family and Medical Leave Act (FMLA) of 1993, employers were already providing maternity leave in response to the increase in the number of women workers. Employers did create maternity-leave programs in the 1970's and 1980's, but not as a purely voluntary response in the absence of any government mandate. In 1972, the Equal Employment Opportunity Commission (EEOC) ruled that employers who allowed leaves for disabling medical conditions must also allow them for maternity and that failure to do so would constitute sex discrimination under the Civil Rights Act of 1964. As early as 1973, a survey found that 58 percent of large employers had responded with new maternity-leave policies. Because the 1972 EEOC ruling was contested in court, the ruling won press attention that popularized maternity-leave policies. Yet perhaps because the Supreme Court later struck down the ruling, politicians and scholars have failed to recognize its effects, assuming that employers adopted maternity-leave policies in response to the growing feminization of the workforce.