Tyler Cowen asks whether children brought up in the same family share the same environmental influences and then concludes “Maybe not”. Maybe not? Definitely not.
He’s referring to a 1987 paper with a recent commentary:
For most psychological characteristics, correlations for adoptive ‘‘siblings’’ hover near zero, which implies that the relevant environmental influences are not shared by children in the same family. Although it has been thought that cognitive abilities represent an exception to this rule, recent data suggest that environmental variance that affects IQ is also of the nonshared variety after adolescence.
My favorite part of the paper is the section which discusses how siblings represents a non-shared environment for each other. For instance my sister grew up with a slightly younger brother and I grew up with…a slightly older sister, which is a somewhat different proposition.
Remember that you are the sum of the experiences of your life, and you and members of your family will never have the same life as you. Little events build up to become big influences on a life. In one experiment, scientists took genetically identical mice and put them in a Big Brother-esque box in order to watch how their behavior changed with time. Tiny, potentially random differences in life snowballed into large differences in personality. Mice that explore a little at the beginning of their life are soon roaming widely across their world later in life, and mice that have no need to venture out at the beginning of their life never will later on.
To take a simple example from my life: when I was growing up, my mom started biking to work around the time that I moved to a new elementary school. My school was in biking distance so obviously I wanted to bike to school just like my mom biked to work. The habit stuck and I’ve biked everywhere since then. By the time my siblings were old enough to be able to bike to school, my mom’s biking had been curtailed and the novelty had worn off. Neither ever biked to school and still don’t. This all means I grew up in a bike-based environment and my siblings grew up in a car-based one – very different. It’s a just-so story but it illustrates the magnification of tiny events that become our divergent lives.
Basically: learning happens.