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Post-Traumatic Stress Disorder

Left-Handedness and Mixed-Handedness in PTSD

New study shows that mixed-handedness is more common in PTSD.

Key points

  • A new meta-analysis investigated mixed-handedness and left-handedness in PTSD patients compared to controls.
  • PTSD patients were more likely to be mixed-handed (19%) than the general population (9%).
  • Brain development could be the factor that links mixed-handedness and PTSD.

Most people identify themselves as either left-handed or right-handed, but scientific evidence suggests that there is a third category for handedness: mixed-handedness. Mixed-handed people do not have a clear preference for one hand but use each hand for different tasks. For example, they may write and draw with their right hand, but use scissors with their left hand. Interestingly, it has been shown that mixed-handedness is overrepresented in some psychiatric disorders, like schizophrenia (Tran and Voracek, 2015). A new study, recently published in the scientific journal Neuroscience and Biobehavioral Reviews focused on investigating mixed-handedness and left-handedness in PTSD (post-traumatic stress disorder) patients (Borawski et al., 2023).

A meta-analysis of handedness in PTSD patients

Since many scientific studies on handedness in PTSD patients have been published over the years, it was possible to conduct a so-called meta-analysis. A meta-analysis is a statistical analysis in which the data from many published studies are analyzed together. The main advantages of a meta-analysis are that methodological problems that can occur in a single study do not affect the data as much and the results are more robust. In short, meta-analyses provide very reliable results that can be trusted to a greater extent than the findings of single studies.

In the study, the scientists analyzed data from 14 studies that provided handedness data for 747 PTSD patients and 2,192 controls without PTSD. Three different analyses were calculated, one for left-handedness, one for mixed-handedness, and one for left-handedness and mixed-handedness combined.

Mixed-handedness is more common in PTSD patients than in the general population

The analyses revealed an intriguing result. The scientists found out that patients with PTSD were significantly more likely to not be right-handed compared to controls. About 29 percent of PTSD patients were non-right-handed in the analysis that combined left-handedness and mixed-handedness. However, when PTSD patients and controls were compared regarding left-handedness, there was no difference. This strongly suggests that the effect of non-right-handedness was mainly driven by differences in mixed-handedness. Indeed, the analysis of mixed-handedness showed a strong and statistically significant result. Overall, about 19 percent of PTSD patients were mixed-handed. In comparison, about 9 percent of the general population is mixed-handed. Thus, the percentage of mixed-handedness in PTSD is more than double the percentage of mixed-handedness in the general population.

Why is there a link between mixed-handedness and PTSD?

This intriguing finding leaves the question: Why? Two possibilities are thinkable.

On the one hand, experiencing trauma and suffering from PTSD could somehow affect the organization of motor networks in the brain and change handedness. This explanation is implausible. In most people, handedness is determined before birth (see my post explaining this phenomenon here). Changes in later life happen only very rarely. The experience of trauma that elicits PTSD happens much later in life than the time point at which handedness is determined in most people.

On the other hand, mixed-handedness may be associated with genetic factors or aspects of brain organization that represent a risk factor for developing PTSD if someone experiences severe trauma later in life. This idea would align with the finding that mixed-handedness is also overrepresented in other disorders, like schizophrenia. Moreover, a large-scale genetic study has shown that the genetic factors that determine handedness overlap to some extent, with the factors involved in several different psychiatric disorders like schizophrenia and bipolar disorder (Cuellar-Partida et al., 2021). While this link between handedness and psychiatric disorders is not well understood, the relevant genes are involved in brain development, suggesting that both handedness and the risk to develop a disorder are influenced by the same processes that affect brain structure.

Taken together, the findings suggest that future PTSD studies should include handedness, as it may be informative of the risk to develop the disorder.

References

Borawski J, Papadatou-Pastou M, Packheiser J, Ocklenburg S. (2023). Handedness in post-traumatic stress disorder: A meta-analysis. Neurosci Biobehav Rev, 145, 105009.

Cuellar-Partida G, Tung JY, Eriksson N, Albrecht E, Aliev F, Andreassen OA, Barroso I, Beckmann JS, Boks MP, Boomsma DI, Boyd HA, Breteler MMB, Campbell H, Chasman DI, Cherkas LF, Davies G, de Geus EJC, Deary IJ, Deloukas P, Dick DM, Duffy DL, Eriksson JG, Esko T, Feenstra B, Geller F, Gieger C, Giegling I, Gordon SD, Han J, Hansen TF, Hartmann AM, Hayward C, Heikkilä K, Hicks AA, Hirschhorn JN, Hottenga JJ, Huffman JE, Hwang LD, Ikram MA, Kaprio J, Kemp JP, Khaw KT, Klopp N, Konte B, Kutalik Z, Lahti J, Li X, Loos RJF, Luciano M, Magnusson SH, Mangino M, Marques-Vidal P, Martin NG, McArdle WL, McCarthy MI, Medina-Gomez C, Melbye M, Melville SA, Metspalu A, Milani L, Mooser V, Nelis M, Nyholt DR, O'Connell KS, Ophoff RA, Palmer C, Palotie A, Palviainen T, Pare G, Paternoster L, Peltonen L, Penninx BWJH, Polasek O, Pramstaller PP, Prokopenko I, Raikkonen K, Ripatti S, Rivadeneira F, Rudan I, Rujescu D, Smit JH, Smith GD, Smoller JW, Soranzo N, Spector TD, Pourcain BS, Starr JM, Stefánsson H, Steinberg S, Teder-Laving M, Thorleifsson G, Stefánsson K, Timpson NJ, Uitterlinden AG, van Duijn CM, van Rooij FJA, Vink JM, Vollenweider P, Vuoksimaa E, Waeber G, Wareham NJ, Warrington N, Waterworth D, Werge T, Wichmann HE, Widen E, Willemsen G, Wright AF, Wright MJ, Xu M, Zhao JH, Kraft P, Hinds DA, Lindgren CM, Mägi R, Neale BM, Evans DM, Medland SE. (2021). Genome-wide association study identifies 48 common genetic variants associated with handedness. Nat Hum Behav 5, 59-70.

Tran US, Voracek M. (2015) Schizophrenia and mixed-handedness. Br J Psychiatry 207, 178.

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