 A seven-year-old arrives home with her first report card from school, which is full of terrible grades. Her dad leans over her shoulder to take a peek at the academic horror show. The girl turns around and asks, so what do you think, dad? Nature or nurture? Centuries ago, John Locke and others began arguing that our personality is mainly shaped by the environment we were raised in. Others, such as Charles Darwin, believe that we are largely programmed genetically. Modern behavioral geneticists think that it's more complex because the two interact. And relatively recently, a little crayfish from Germany showed just how little we actually know, but more about her later. First, let's go back to the girl, her dad, and what we do know. Neither the dad nor the smartest scientists can answer what leads to certain school outcomes of our seven-year-old girl. We do know, however, a bit about the influences of genes and the environment when it comes to groups of specific populations. Take for example, a group of a hundred white American men in their early twenties, and you will probably find, on average, an IQ of a hundred, a height of 178 centimeters or around 5.8 feet, and that ten have spelling problems. We can now ask, which of these three traits is determined by genes, anything that happened before conception often referred to as nature, and which by the environment, anything that happened after their mothers got pregnant, often referred to as nurture? To understand the two forces, we need to acknowledge that environmental influences are often random, siblings experience home very differently, and we need to recognize that genes are complex, there is no single gene doing a single thing. We therefore know little about which trait in an individual is the result of their genetic makeup and which is due to the environment. We only know about the impact of nature and nurture on trait differences of a particular group, and to learn about that, we need to understand heritability. Heritability is a factor that ranges between one and zero. One stands for genetic influences, zero for environmental factors. Heritability defines the genetic impact on trait differences in a particular population. It cannot measure the degree to which a trait of a particular individual is genetic. Dyslexia, for example, has a heritability closer to one. That means that genes explain more of the difference in spelling mistakes of our group than school, family, or random factors. It does not mean that one particular person with spelling problems got them genetically, some just didn't get much support when they were young. How complex the interaction between genes and environment really is becomes clear when we look at something as seemingly straightforward as IQ and height. First, let's examine height. A group of 100 white American male students are on average 178 cm tall, that's roughly 5'10". Their heritability for height is around 0.8. If some men in this group are 183 cm or 6' tall, then heritability allows us to assume that 80% of that difference is, on average, caused by genetic variance, and 20% of the difference is due to lifestyle. If the same group of people were raised in a region that experienced systemic droughts and they never had enough to eat, their potential for an average height of 178 cm is far from ever being reached. The heritability for height might now go from 0.8 down to 0.5. Nature and nurture are now equally responsible for group differences. This means heritability changes. If we have a well-nourished population on the one side and a malnourished group on the other, then the impact of nutrition on the differences in height changes from 20% to 50%. In other words, nurture matters a lot until we reach a point from which it's not as relevant. So what about intelligence? The heritability of IQ is around 0.6 for people in their 20s and then increases as we age. The average IQ in our group will be around 100, but some have an IQ of 110. These are therefore responsible for 60% of the difference and random factors in the environment for about 40%. In other words, for those with an IQ of 110, 6 of the additional 10 points can, on average, be attributed to nature and 4 points to nurture. Now even if nurture plays a big role, it doesn't mean that teachers or parents had any impact. Unlike height, which we know how to increase through nutrition, when it comes to IQ we don't know much about how to improve it. Random factors seem to play a big role. How random the environment is becomes clear when we look at two kids that are born into the same family. Both are raised in the same shared environment, but first-borns, on average, turn out to be more intelligent than their younger siblings. And if one of the two siblings was adopted, the two would have some correlation in IQ during their upbringing. But as adults, they would be hardly more similar in intelligence than complete strangers. Parents, it seems, have a very limited impact on a child's IQ. So now what about that crayfish? In 1995, German scientists made an odd observation. The little female creature had mutated and become asexual, able to make clones of itself. As each clone shared exactly the same genetic blueprint, the scientists set up an experiment for the ultimate test. Hundreds of these little identical creatures were placed in the same environment. Same water, same temperature, same amount of food. Even though the scientists tried to control all these variables, the unthinkable happened. Some crayfish stayed tiny, others grew big, some died, others lived on and on, and while some became social, others enjoyed solitude. Why do you think that happened? 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