Exploring the Impact of Genetics and Environment on Weight
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Understanding the Nature vs. Nurture Debate
You may find the topic of weight management more complex than you initially thought!
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In February 2020, I shared an article on Medium titled "Thin Individuals Shouldn't Offer Weight Loss Guidance."
Thin Individuals Shouldn't Offer Weight Loss Guidance
Or, the concept of illusory correlation in practice!
This piece wasn't intended to shame those who are slim; instead, it aimed to illustrate the concept of illusory correlation. The American Psychological Association describes illusory correlation as “the perception of a connection that does not genuinely exist.”
The core argument of the article reflects my perspective that many slender individuals attribute their physiques to their daily habits or diets—essentially their lifestyle—when, in fact, these factors may have little to do with their slenderness.
My conclusion stemmed from sharing a living space with a slim person while I struggled with significant obesity. We consumed the same meals and engaged in similar activities, yet her BMI hovered around 21 while mine was nearly 48. Though many elements contribute to BMI, if I weren't more than double her figure, I would delve into those details...
While I provided some scientific support for my observations, the article was fundamentally an opinion piece. Recently, I began reading "Blueprint" by Robert Plomin, a leading figure in behavioral genetics, which seeks to clarify the nature versus nurture debate concerning various human traits. Specifically, it examines what influences individual differences in psychological and behavioral characteristics.
So, what determines a person's cheerful disposition—genetic factors or environmental influences?
I haven't progressed far into the book, but the opening chapter discusses why certain individuals are overweight while others are not. Plomin analyzes how researchers assess the extent to which traits are shaped by genetics versus environmental factors, relying significantly on studies involving twins and adoptees.
Twin Research Methodologies
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When studying twins, researchers often differentiate between monozygotic (identical—MZ) twins and dizygotic (fraternal—DZ) twins. MZ twins share 100% of their DNA, while DZ twins share approximately 50%, akin to siblings born years apart.
A common analytical approach is Correlation Analysis, which determines if two variables exhibit similar trends. Nutrition scientists, for example, may explore the relationship between chocolate consumption and body weight, assessing whether increased chocolate intake correlates with higher weight. This correlation can range from 0 to 1.0 for a positive association (more chocolate leads to more weight) or from -1.0 to 0 for a negative association. However, a strong correlation should not be misinterpreted as one variable causing the other; the phrases "correlation does not imply causation" often come into play here.
In behavioral genetics, correlation analysis is applied differently. Researchers examine whether a relationship exists between two groups based on one variable—namely, weight. They don't seek to establish causation; rather, the strength of the relationship contributes to the broader understanding of the puzzle they are investigating.
If body weight were solely determined by genetics, we would anticipate a correlation of 1.0 for weight between MZ twins since they share identical DNA.
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Conversely, if weight were entirely genetic, we would expect the correlation between DZ twins to approximate 0.5, given their 50% genetic overlap. To make the analysis more consistent with the MZ comparison, only same-sex DZ twins were considered (as MZ twins are always the same sex).
The findings? The correlation for weight among MZ twins across different ages is 0.75, indicating a significant genetic influence on body weight. The correlation for DZ twins is 0.55, further supporting the notion of a strong genetic component, albeit lower than that of MZ twins.
However, these twins were raised in the same environment—sharing the same household, parents, lifestyle, and dietary habits. Fortunately, data sets exist for twins who were adopted and raised separately, providing valuable insights into the nature-nurture debate regarding weight.
Dr. Plomin analyzed data from a Swedish dataset of MZ twins raised apart, discovering a weight correlation of 0.73. Another dataset from the Minnesota Study of Twins Raised Apart yielded the same correlation. Overall, the average correlation across various datasets of MZ twins raised apart is 0.75, reinforcing the argument that genetics significantly influences body weight, even if environmental factors also play a role.
The Role of Environment
The aforementioned results indicate a significant environmental aspect regarding weight. Even MZ twins, despite sharing identical DNA, can have different environments and consequently different weight influences. For instance, one family might adhere to a healthy diet while another indulges in junk food. Yet, the correlation in weight for MZ twins remains consistent whether they were raised together or apart.
Behavioral genetics suggests that our understanding of nurture might need reevaluation. The aspect of nurture responsible for variations in MZ twin weight—and indeed all human traits—may not align with our traditional beliefs.
Behavioral genetics identifies systematic aspects of upbringing—such as lifestyle, socio-economic status, family dynamics, and community—as part of the "shared environment." In this context, MZ twins raised together share the same environment, while those raised apart have different environments (each shares an environment with their adoptive families). The consistent correlation of 0.75 for MZ twins raised together and apart implies that this shared form of nurture isn't the driving factor behind weight differences.
This observation has given rise to the concept of a "non-shared environment," which accounts for the individual differences not explained by genetics. In the context of the research discussed, the non-shared environment encompasses random variances in the lives of these twin pairs. For the past 30 years, behavioral genetics has sought to pinpoint common threads within this non-shared environment, yet to no avail.
Additional Insights
There are intriguing weight-related statistics from intact families and studies involving adopted children and their adoptive parents.
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The correlation between the weight of birth mothers and their adult children stands at 0.30. Though this is lower than that of DZ twins, it's expected since the non-shared environments of mothers and children differ significantly compared to DZ twins. However, when comparing adopted children with their adoptive mothers (with no shared DNA), the correlation is 0.0. Yet, when comparing the weights of adopted children with their biological mothers, the correlation rises to 0.3—even though their environments were entirely different.
These findings strongly suggest that weight is largely influenced by our genetic makeup. This reinforces my assertion that individuals who have never grappled with weight issues likely possess DNA that efficiently regulates their body weight. My somewhat tongue-in-cheek viewpoint regarding thin individuals providing weight loss advice is further validated by behavioral genetics.
Interestingly, I did not seek this validation actively. I frequently tune into Sam Harris’s podcast "Making Sense," where Dr. Plomin discussed the book on behavioral genetics I mentioned earlier.
I appreciate your time in reading this article—hopefully, you found it insightful!
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The first video titled "Is Weight Determined By Nature or Nurture?" explores the ongoing debate about the factors affecting body weight.
The second video, "Is It Nature Instead of Nurture That Makes You, You?" features Robert Plomin discussing the genetic aspects that shape individual identities.