12.14
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, chron…
Characteristics like height are not only shaped by several genes referred to as the influence of nature but also by various environmental factors collectively called nurture.
For example, poor nutrition during childhood such as a lack of protein can stunt an individual's growth causing them to be shorter at adulthood than they would have been with better nutrition. Interestingly, improvements in nutrition likely contributed to a gradual increase in average height since the early 20th century. A trend particularly evident in certain regions of the Americas, Europe and Asia.
However, evidence also exists that children's emotional environments like chronic stress and a lack of socialization interferes with levels of growth hormones in the body decreasing them and similarly curbing stature. So, sometimes it's the interplay between nature and nurture that dictates the final appearance of a phenotype like someone's height and even whether that person develops cancer or heart disease later in life.
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Q1: What percentage of height variation is determined by genetics versus environment?
Scientists estimate that 70-90% of variation in height is due to genetic differences among individuals, while 10-30% results from environmental differences such as diet and living conditions. This demonstrates that height is a human height polygenic trait influenced by both nature and nurture, with genes playing the dominant but not exclusive role in determining stature.
Q2: How does chronic stress during childhood affect growth and height?
Chronic stress inhibits physical growth through the hypothalamic-pituitary-adrenal axis. Stress hormones like cortisol reduce the liver's production of IGF-1, needed for bone growth, and increase somatostatin release, which inhibits growth hormone. Children experiencing chronic stress show reduced bone growth and weight gain, though these effects can resume when stress is eliminated.
Q3: What role does nutrition play in determining adult height?
High-quality animal protein intake during childhood is the most important nutritional factor explaining differences in stature among populations. Poor nutrition, particularly protein deficiency, stunts growth and reduces adult height. Improved nutrition since the early 20th century has contributed to gradual increases in average height, particularly evident in the Americas, Europe, and Asia.
Q4: How do emotional environments and social relationships influence childhood growth?
Children raised under socially stressful conditions with inadequate physical and emotional support experience growth delays. Institutionalized children lacking strong, consistent caregiver relationships show stunted growth. Psychosocially-induced reductions in growth hormone combined with unmet emotional needs directly inhibit bone development and weight gain during critical developmental periods.
Q5: Why is it important to consider both nature and nurture when studying phenotypes?
Many phenotypes, including height and disease susceptibility, result from the interplay between inherited genes and environmental conditions. People inherit different genes and develop under different environmental circumstances, making it essential to examine both factors. Ignoring either nature or nurture provides an incomplete understanding of how characteristics and diseases develop.
Q6: What evidence shows that improved living conditions have increased human height over time?
Upper Paleolithic hunters from the Gravettian culture (34,000-26,000 years ago) reached heights of 5'10" to 6'2", attributed to low population density and abundant high-quality animal protein. In modern times, increased stature is attributed to nutritional and socioeconomic improvements. This historical comparison demonstrates how environmental factors like diet and resource availability directly influence population-level height changes.
Q7: Can growth stunting from stress and poor nutrition be reversed in children?
Yes, growth stunting can be reversed when stress is reduced or eliminated and nutrition improves. Children experiencing chronic stress show inhibited bone growth and weight gain, but these effects resume when stressful conditions end. This reversibility demonstrates that environmental factors are not permanently fixed and that improving living conditions can restore normal growth trajectories.