<P> It has been observed that first - born males are shorter than later - born males . However, more recently the reverse observation was made . The study authors suggest that the cause may be socio - economic in nature . </P> <P> The precise relationship between genetics and environment is complex and uncertain . Differences in human height is 60--80% heritable, according to several twin studies and has been considered polygenic since the Mendelian - biometrician debate a hundred years ago . A genome - wide association (GWA) study of more than 180,000 individuals has identified hundreds of genetic variants in at least 180 loci associated with adult human height . The number of individuals has since been expanded to 253,288 individuals and the number of genetic variants identified is 697 in 423 genetic loci . In a separate study of body proportion using sitting - height ratio, it reports that these 697 variants can be partitioned into 3 specific classes, (1) variants that primarily determine leg length, (2) variants that primarily determine spine and head length, or (3) variants that affect overall body size . This gives insights into the biological mechanisms underlying how these 697 genetic variants affect overall height . These loci do not only determine height, but other features or characteristics . As an example, 4 of the 7 loci identified for intracranial volume had previously been discovered for human height . </P> <P> The effect of environment on height is illustrated by studies performed by anthropologist Barry Bogin and coworkers of Guatemala Mayan children living in the United States . In the early 1970s, when Bogin first visited Guatemala, he observed that Mayan Indian men averaged only 157.5 centimetres (5 ft 2 in) in height and the women averaged 142.2 centimetres (4 ft 8 in). Bogin took another series of measurements after the Guatemalan Civil War, during which up to a million Guatemalans fled to the United States . He discovered that Maya refugees, who ranged from six to twelve years old, were significantly taller than their Guatemalan counterparts . By 2000, the American Maya were 10.24 cm (4.03 in) taller than the Guatemalan Maya of the same age, largely due to better nutrition and health care . Bogin also noted that American Maya children had relatively longer legs, averaging 7.02 cm (2.76 in) longer than the Guatemalan Maya (a significantly lower sitting height ratio). </P> <P> The Nilotic peoples of Sudan such as the Shilluk and Dinka have been described as some of the tallest in the world . Dinka Ruweng males investigated by Roberts in 1953--54 were on average 181.3 centimetres (5 ft 11 ⁄ in) tall, and Shilluk males averaged 182.6 centimetres (6 ft 0 in). The Nilotic people are characterized as having long legs, narrow bodies and short trunks, an adaptation to hot weather . However, male Dinka and Shilluk refugees measured in 1995 in Southwestern Ethiopia were on average only 176.4 cm and 172.6 cm tall, respectively . As the study points out, Nilotic people "may attain greater height if privileged with favourable environmental conditions during early childhood and adolescence, allowing full expression of the genetic material ." Before fleeing, these refugees were subject to privation as a consequence of the succession of civil wars in their country from 1955 to the present . </P>

What was the average height of a man in the 1800s