What Causes Left-Handedness? – Scientific Theories Explained Simply

About 10% -20% of all people are left-handed – but why? Researchers have developed several theories to explain handedness. This overview presents the most important scientific ideas in a way that’s easy to understand – perfect for school projects and curious minds.

1. Genetic Theories: Is Left-Handedness Inherited?

Scientists believe that genetics play a role in handedness. The “Right-Shift Theory” by Marian Annett and the “Dextral/Chance Model” by I.C. McManus suggest that some genes influence whether a person becomes right- or left-handed. However, no single “left-handed gene” has been clearly identified. Studies estimate that genes account for around 25% of handedness, with the remaining influence coming from environmental factors. [source]

2. Hormonal Influences: The Geschwind-Behan-Galaburda Hypothesis

In the 1980s, neurologists Norman Geschwind, Peter Behan and Albert Galaburda developed a theory that high levels of testosterone in the womb could delay the development of the left brain hemisphere. As a result, the right hemisphere becomes more dominant, which could lead to left-handedness. This theory also tries to explain links between left-handedness and language or immune-related conditions. However, many researchers criticize this hypothesis due to lack of consistent data. [source]

3. Prenatal Environment: External Influences Before Birth

Some researchers, like F.H. Previc, suggest that factors such as the baby’s position in the womb may influence handedness. If one hand gets more stimulation than the other before birth, this could shape brain development and hand preference later in life. This theory highlights the role of the prenatal environment rather than genes alone. [source]

4. Epigenetics: When Genes and Environment Interact

Epigenetics is the study of how the environment can affect how genes are activated without changing the genetic code. Biologist Conrad Hal Waddington coined the term. In terms of handedness, epigenetic factors might influence how gene expression guides motor preferences like hand use. This approach supports the idea that both nature and nurture matter. [source]

5. A Critical Perspective

Despite many theories, science hasn’t found one clear answer to explain left-handedness. Most researchers agree that it results from a combination of genetic, hormonal, and environmental factors. However, the data is often mixed or incomplete, and more research is needed to understand the exact causes.

Conclusion

Left-handedness is a natural variation in human development. While several scientific theories exist, none fully explain why some people are left-handed. The most likely answer lies in a mix of biology, hormones, and individual development before birth.