Studies in modern times have observed a gradual increase in height with maternal age, though these early studies suggest that trend is due to various socio-economic situations that select certain demographics as being more likely to have a first birth early in the mothers life. These same studies show that children born to a young mother are more likely to have below-average educational and behavioural development, again suggesting an ultimate cause of resources and family status rather than a purely biological explanation.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. Nature versus nurture
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.
Urban mortality was also much higher than in rural regions. In 1829, the average urban and rural Dutchman was 164 cm (5 ft 4+1⁄2 in). By 1856, the average rural Dutchman was 162 cm (5 ft 4 in) and urban Dutchman was 158.5 cm (5 ft 2+1⁄2 in).A 2004 report citing a 2003 UNICEF study on the effects of malnutrition in North Korea, due to "successive famines," found young adult males to be significantly shorter. In contrast South Koreans "feasting on an increasingly Western-influenced diet," without famine, were growing taller. The height difference is minimal for Koreans over forty years old, who grew up at a time when economic conditions in the North were roughly comparable to those in the South, while height disparities are most acute for Koreans who grew up in the mid-1990s – a demographic in which South Koreans are about 12 cm (4.7 in) taller than their North Korean counterparts – as this was a period during which the North was affected by a harsh famine where hundreds of thousands, if not millions, died of hunger. A study by South Korean anthropologists of North Korean children who had defected to China found that eighteen-year-old males were 13 centimetres (5 in) shorter than South Koreans their age due to malnutrition.The tallest living man is Sultan Kösen of Turkey, at 251 cm (8 ft 3 in).