For centuries, theorists have debated what contributes to human behavior, is it how we are born or is it what we learn. John Locke (1632-1704) had a theory called Tabula Rasa or blank slate, which argued that nothing was innate and it was all learned. This perspective influenced social sciences for many years going forward. Gregor Mendel (1822-1884) associated with the development of genetics as a theory, spawned significant amount of research and theorizing about how genetics influenced human development, however it would be many years before human DNA was sequenced. During the Third Reich, Nazis murdered those with disabilities, those who were different, gay, Jewish, gypsies and communists thanks to beliefs in eugenics. The Nazis thought they were superior and many of the others were inferior racially. This is an example of how science can go terribly wrong.
Twins Studies have been considered the gold standard for considering if traits are genetically predetermined. Monozygotic (MZ) or identical twins share 100% of their DNA as they were from the same egg. Dizygotic (DZ) twins share less common DNA. Comparing identical twins living together and who were raised apart and the differences between MZ and DZ twins, helped develop estimates of heredity. In an article in a professional psychological journal Wendy Johnson, Eric Turkheimer, Irving I. Gottesman, and Thomas J. Bouchard, Jr. authored “Beyond Heritability–Twin Studies in Behavioral Research” in which they suggested that time has come for abandoning twin studies in research into genetic influences. It has become clearer and clearer that environment poses many confounding variables in proving causation. As one would hear in a research class, “Correlation does not mean causation” that just because there is a relationship between two factors, does not mean that one causes the other. There can be many variables that could interfere or confound or cloud research into the relationship. Instead of arguing that A causes B, B could cause A or perhaps they interact. One of the realities that many of us are such a large mixture of various genes and influences that it is hard to know who’s who, and what I got from whom.
After much research, the current state of research suggests estimates of the heredity of Intelligence are in the vicinity of 50 to 80%. Current thinking suggests that genetics interacts with environment in terms of Genes x Environment rather than Genes + Environment. The interaction leads to genetic expression in terms of proteins and amino acids and others, which influence the outcome.
In another example of how science can go wrong (there are many examples), the book published in 1995, the Bell Curve by Harvard Psychologist Richard J. Hernstein and American Enterprise Institute political science researcher Henry Murray suggested that genetics contribute to far more than one would expect. Including African Americans are less intelligent than white people because of genetic differences. This smells a lot like Eugenics to me. This generated a lot of controversy and seems rather questionable. Another book from other scientists was published to refute those findings “Intelligence, Genes, and Success: Scientists Respond to THE BELL CURVE (Statistics for Social Science and Public Policy)” Edited By: Bernie Devlin, Stephen E. Fienberg, Daniel P. Resnick, and Kathryn Roeder. The American Psychological Association formed a task force and developed their official statement on the matter arguing that race, gender are not correlated to intelligence, but intelligence is related to many factors and influences.