Summary
Highlights
Centuries ago, philosophers like John Locke believed personality was shaped by environment (nurture), while others like Charles Darwin thought it was genetically programmed (nature). Modern scientists believe it's a complex interaction between the two, a concept that a little crayfish from Germany further illuminated.
While we can't definitively say what causes traits in individuals, we can understand the influences on groups. Environmental influences are often random, even for siblings, and genes are complex, not single-trait specific. We study the impact of nature and nurture on 'trait differences' within a group, which leads to the concept of heritability.
Heritability ranges from 0 to 1; 1 represents genetic influence and 0 represents environmental factors. It defines the genetic impact on trait differences in a particular population, but it *cannot* measure how genetic a trait is in an individual. For example, dyslexia has high heritability, meaning genes explain more of the group difference in spelling mistakes than environment, but it doesn't mean an individual's dyslexia is purely genetic.
For a group of white American men, the heritability for height is around 0.8. This means 80% of height differences are attributed to genetics and 20% to lifestyle. However, in a malnourished population, heritability can drop to 0.5, showing that nurture becomes equally responsible for group differences. This demonstrates that heritability is not fixed.
The heritability of IQ is around 0.6 for people in their twenties, increasing with age. This means 60% of IQ differences in a group are due to genes and 40% to random environmental factors. While nurture plays a role, we don't fully understand how to improve IQ, and random factors seem significant. Parents appear to have limited impact on a child's IQ; studies show firstborns tend to be more intelligent, and adopted siblings' IQ similarity diminishes in adulthood.
In 1995, German scientists observed a female crayfish mutate to become asexual, producing genetic clones. When hundreds of these identical clones were raised in the exact same controlled environment (water, temperature, food), they still showed diverse outcomes in size, lifespan, and social behavior. This experiment highlights the complex interplay of nature, nurture, and random variations, even with identical genetic codes.