How R.A. Fisher's Revolution Created Behavioral Genetics
Imagine a scientific world divided by two warring camps—one believing that complex human traits like intelligence and personality were shaped by many genes working together, the other convinced that single genes determined our destiny. This was the state of genetics in the early 20th century, a field at war with itself until Ronald Aylmer Fisher, a brilliant British statistician and geneticist, built a bridge between these opposing worlds. His foundational work in the early 1900s didn't just resolve a scientific crisis—it created entirely new disciplines, including what we now know as behavioral genetics.
Fisher's 1918 paper, "The Correlation Between Relatives on the Supposition of Mendelian Inheritance," was nothing short of revolutionary. It demonstrated mathematically that the competing theories of biometry (which studied continuous variation) and Mendelism (which focused on discrete inheritance) weren't contradictory but complementary 9 .
This synthesis allowed scientists to study how genes influence complex behavioral traits that don't follow simple inheritance patterns. Fisher provided the statistical toolkit that would enable researchers to explore the genetic architecture of behavior, launching a century-long investigation into how our genes make us who we are.
Fisher's genius lay in developing sophisticated mathematical frameworks to tackle seemingly intangible questions about heredity and behavior. Three contributions in particular would become the bedrock of behavioral genetics.
Fisher resolved the conflict between Mendelians and biometricians by showing that continuous variation could be explained by many genes following Mendelian principles 9 .
Fisher developed analysis of variance (ANOVA) to partition behavioral variation into genetic and environmental components, enabling calculation of heritability 7 .
Fisher's fundamental theorem of natural selection provided a mathematical approach to understanding how evolution shapes behavioral adaptations 5 .
Fisher's statistical innovations gave birth to conceptual frameworks that continue to guide behavioral genetics research today. These principles have proven remarkably durable across decades of scientific discovery.
One of the most robust findings to emerge from the Fisherian tradition is what some have called the "first law of behavioral genetics": All behavioral traits show significant and substantial genetic influence 2 .
A landmark 2015 meta-analysis of virtually every twin study conducted over 50 years—covering more than 18,000 traits and 15 million twin pairs—found that all behavioral traits are heritable to some degree 2 9 . The average heritability estimate for behavioral traits hovers around 50% 9 .
Fisher's quantitative framework set the stage for a genomic revolution that has transformed how we study the genetics of behavior. What began with mathematical models applied to family studies has evolved into the direct analysis of DNA.
Genome-wide association studies (GWAS) represent the ultimate fulfillment of Fisher's vision—analyzing thousands of individual genetic variants across the genome to identify those associated with specific behavioral traits 9 .
Polygenic scores aggregate the effects of thousands of genetic variants to predict genetic predisposition for behaviors, representing the practical application of Fisher's polygenic model 9 .
First twin & adoption studies - Preliminary evidence of genetic influence on intelligence
Larger twin studies - Recognition of universal heritability
Model-fitting approaches - Discovery of nonshared environment
Genome-wide association studies - Identification of specific genetic variants associated with behavior
To understand how Fisher's methods transformed behavioral science, we need look no further than the landmark 1976 twin study by Loehlin and Nichols that first revealed the "first law" of behavioral genetics.
The researchers applied the classical twin design, which compares the similarity of identical (monozygotic) twins, who share nearly 100% of their genes, with fraternal (dizygotic) twins, who share approximately 50% of their segregating genes.
Fisher's analytical framework allowed them to partition the variance in numerous behavioral measures into genetic components, shared environmental components (experiences that make twins similar), and nonshared environmental components (experiences that make twins different), plus measurement error.
The findings were startling in their consistency. Across virtually every measure, identical twins proved significantly more similar than fraternal twins. The researchers discovered what they called a "curious uniformity of identical-fraternal differences both within and across trait domains" 9 .
Perhaps even more surprisingly, the study revealed that shared family environment accounted for surprisingly little of behavioral similarity.
| Trait Category | MZ Correlation | DZ Correlation | Estimated Heritability |
|---|---|---|---|
| Extraversion | 0.52 | 0.18 | ~68% |
| Neuroticism | 0.47 | 0.20 | ~54% |
| Social attitudes | 0.66 | 0.42 | ~48% |
| Intellectual interests | 0.64 | 0.34 | ~60% |
"Environment carries substantial weight in determining personality—it appears to account for at least half the variance—but that environment is one for which twin pairs are correlated close to zero" 9 .
Modern behavioral genetics continues to rely on methods and concepts derived from Fisher's original work. Here are the key tools in the behavioral geneticist's toolkit:
A century after Fisher's groundbreaking work, his influence permeates every corner of behavioral genetics. What began as abstract mathematical models has evolved into a sophisticated science that continues to reveal the complex origins of human behavior. The Fisherian tradition reminds us that the nature-nurture debate was always based on a false dichotomy—our behaviors emerge from dynamic interactions between genetic predispositions and life experiences.
As we enter the era of behavioral genomics, where we can examine DNA directly rather than inferring genetic influence from family relationships, Fisher's insights seem more relevant than ever. The Fisherian tradition has given us more than just answers—it has provided a robust framework for asking better questions about what makes us who we are.