Letter from the Dean

Nature, nurture, sex and gender

Understanding our differences points the way to precision health

It’s a long-standing debate: Are individual differences the result of our genes or our environment? Nature or nurture? For psychologists seeking to understand why we behave as we do, and for scientists and physicians looking for the underlying causes of disease and illness, it’s a vitally important question.

Photograph by Glenn Matsumura

In recent years, both sides have capitulated to what seems like an obvious compromise: It’s both. Our genes and our environment play leading roles in shaping who we are. But to Siddhartha Mukherjee, physician and author of The Gene, this compromise is “an armistice between fools.” The answer — nature or nurture — depends on the question.

Take sex and gender. The genes that govern gender identity are hierarchically organized, Mukherjee argues. At the top, nature acts alone. A variation in a single chromosome determines whether our sex is male or female.

Geneticist Nettie Stevens, a Stanford graduate, first came to this conclusion in 1905 based on her pioneering discovery that male mealworms produced sperm with either X or Y chromosomes, while females produced eggs with only X chromosomes. At the time, it was commonly believed that sex was determined by environmental factors, such as maternal nutrition. Stevens showed that sex was determined by nature, and nature alone.

Gender, on the other hand, is determined lower in Mukherjee’s hierarchy. There, genes interact continually with the forces of history, society and culture, making gender and gender identity not an either/or, but a spectrum based on an infinite number of influences and interactions.

Consider that women consistently outlive men in developed countries — a robust finding spanning time, place, religion and political regime. Genes and environment each play an important role, but together they cannot explain the gap.

Looking at mortality data for 187 countries over the past five decades, Stanford Medicine’s Mark Cullen found that women consistently exhibit a greater survival “resilience” to social and environmental adversity. This lends support to the “socio-biologic” explanation. Women live longer because they are hardwired to demonstrate social behaviors that promote survival, such as nesting and family protection. The female survival advantage is not the result of the simple addition of nature plus nurture, but rather of a complex interaction between the two.

Unraveling the complex interplay of cause and effect is at the heart of Stanford’s precision health vision. To keep people healthy, we must first understand the basis of health and disease — to explore nature and nurture in a way that goes beyond the outdated dichotomy and incorporates sex and gender as essential factors influencing individual differences.

Keep reading to learn about some of the ways that we at Stanford Medicine are working to advance the scientific understanding of sex and gender — from nature to nurture and back again — to improve the health of all individuals.

Sincerely,
Lloyd Minor, MD
Carl and Elizabeth Naumann Dean of the School of Medicine
Professor of Otolaryngology-Head and Neck Surgery

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