The Illusion of 'True' Heritability: Unpacking the Lifespan Study
A recent paper published in Science has sparked a debate about how much of our lifespan is determined by our genes. The headline suggests a significant jump in the heritability of human lifespan—from the traditional estimate of 23-35% up to approximately 50%. However, a closer look at the methodology reveals that this isn't a discovery of a new biological fact, but rather a result of how the researchers redefined the parameters of their study.
The Traditional View of Heritability
To understand the controversy, we first need to understand how heritability is typically measured. The standard approach uses twin studies. Because identical (monozygotic) twins share 100% of their DNA and fraternal (dizygotic) twins share roughly 50%, researchers can compare the correlation of a trait between the two groups. If a trait is significantly more correlated in identical twins, it is attributed to genetics.
For lifespan, historical twin data from Danish and Swedish cohorts have consistently placed heritability in the 23-35% range. This means that in the real world—where people die from car accidents, infectious diseases, and lifestyle choices—genetics accounts for about a third of the variance in how long people live.
The 'Twin Simulator' and Extrinsic Mortality
The Science paper takes a different approach. Instead of relying solely on observational data, the researchers built a mathematical model—a "twin simulator"—to estimate the probability of death at any given age.
A key component of this model is a parameter called extrinsic mortality. This represents the risk of death from non-aging-related causes, such as accidents, murder, or sudden infectious disease. The researchers assumed this risk is constant across all individuals and over time.
By manipulating this parameter in their simulation, they discovered a predictable trend: as extrinsic mortality decreases, the heritability of lifespan increases. This is intuitive; if you remove the "noise" of random accidents and external tragedies, the remaining variance in lifespan is more likely to be driven by genetics.
Redefining the World to Find a Number
The central claim of the paper is that if you simulate a hypothetical world where extrinsic mortality is zero—a world where no one ever dies from a car crash or a random infection—the heritability of lifespan rises to about 50%.
This is where the technical nuance becomes critical. The researchers aren't reporting the heritability of lifespan in our world, but the heritability of lifespan in a simulated world. As one critic noted, this is akin to redefining "hair color" to mean "hair color, but ignoring hair dye and the process of going gray." While the result is mathematically sound within the model, it doesn't change the observational reality of human life.
Critical Perspectives and Counterpoints
The discussion surrounding this paper highlights several fundamental tensions in statistical genetics:
1. Heritability is a Ratio, Not a Constant
Heritability is not a fixed biological property of a gene; it is a ratio of genetic variance to total phenotypic variance. As a result, heritability depends entirely on the environment. If every child in a society received identical nutrition and schooling, the heritability of IQ would technically increase because the environmental variance would decrease.
2. The 'Confounding Factor' Debate
Some argue that removing extrinsic mortality is "good science" because it isolates the biological potential for lifespan. From this perspective, accidents are "confounding factors" that obscure the true genetic limit of human life. However, others argue that there is no such thing as "true" heritability independent of the world we actually inhabit.
3. The Gap Between Model and Reality
Even within the paper's own logic, the "50%" figure is an asymptote. When the researchers adjusted the extrinsic mortality parameter to match modern human levels (approximately 0.001 per year), the estimated heritability was actually closer to 35-45%, not 50%.
Why This Research Still Matters
Despite the critiques of its framing, the study provides valuable insights. First, it quantifies how much heritability would increase if we further reduced accidental deaths. Second, it suggests that because modern healthcare has already reduced extrinsic mortality compared to the 19th-century cohorts used in early twin studies, the heritability of lifespan for people alive today is likely higher than the 23-35% previously cited.
Ultimately, the paper serves as a reminder that in science, the definition of the metric is often as important as the result itself. When we ask "how heritable is lifespan?", the answer depends entirely on whether we are asking about the world as it is, or the world as we wish it to be.