12.19: Nature and Nurture
Many human characteristics, like height, are shaped by both nature—in other words, by our genes—and by nurture, or our environment. For example, chronic stress during childhood inhibits the production of growth hormones and consequently reduces bone growth and height. Scientists estimate that 70-90% of variation in height is due to genetic differences among individuals, and 10-30% of variation in height is due to differences in the environments that individuals experience, such as differences in diet. Many other phenotypes are similarly influenced by both genes and environments. Some of these phenotypes arise later in life, like cancer and other diseases.
Nutrition’s Influence on Height
Hundreds of genes that influence height in humans have been identified. One study found that the most important factor explaining current differences in stature among men in different nations was the ratio between the intake of high-quality animal protein and low-quality proteins from grain and vegetables during childhood.
The tallest early men are believed to be Upper Paleolithic hunters from the Gravettian culture, which thrived in Europe 34,000 to 26,000 years ago. Their stature (ranging from 5’10” to 6’2”) was due in part to a low population density and a diet abundant in high-quality animal protein. In modern times, increased stature is attributed to nutritional and socioeconomic improvements.
The Impact of Stress on Stature
Physical growth is metabolically demanding and is inhibited by stress, in part through the hypothalamic-pituitary-adrenal axis. For example, corticotropin-releasing hormone (CRH) stimulates the synthesis of adrenocorticotropic hormone (ACTH), which induces the release of glucocorticoids (e.g., cortisol, the “stress hormone”). Cortisol stimulates appetite but reduces the liver’s production of IGF-1, which is also needed for bone growth and development. Additionally, an excess of glucocorticoids increases somatostatin release, which inhibits the release of growth hormone.
In children, chronic stress has been shown to inhibit bone growth and weight gain—both of which can resume when stress is reduced or eliminated. In addition, when children are raised under socially stressful conditions, where too many children are sharing too few resources, and both physical and emotional needs are unmet, children can also experience growth delay. This is evident in institutionalized children who do not develop strong, consistent relationships with their caregivers. Therefore, stress, psychosocially-induced growth hormone reduction, and inadequate nutrition can all stunt stature.
A Combination of Nature and Nurture
Because people inherit different genes and grow, live, and develop under different environmental conditions, it is important to consider that both nature and nurture influence the final appearance of many phenotypes—even those that may develop later in life, like cancer or heart disease.