Are there two or three forms of handedness?
Most psychological studies investigating handedness have two different categories: left-handedness and right-handedness. A large-scale meta-analysis integrating the results of such studies found that about 10.6% of the volunteers investigated in these studies were left-handed and 89.4% were right-handed (Papadatou-Pastou, and co-workers, 2020).
Interestingly, a minority of handedness studies also included a third handedness category: mixed-handedness. Indeed, a recent study using sophisticated statistical methods confirmed that handedness is indeed is very likely to have three different forms, not two (Mundorf and co-workers, 2024). Typically, psychological handedness studies find out about the handedness of someone using questionnaires containing several questions about their preferred hand for various activities. This could, for example, include writing, drawing, cutting, opening a lid, and other activities. For each activity, the person has to say whether they always use one hand, mostly use one hand, or use both hands equally. Typically, someone who is right-handed would always or mostly use their right hand for all activities. Someone who is left-handed would always or mostly use their left hand for all activities. Compared to left-handers and right-handers, mixed-handers do not have a clear preference for one hand. Typically, the use the left hand for some activities and the right hand for others. For example, a mixed-hander could use the left hand for writing and drawing, but the right hand for throwing a baseball and opening the lid of a jam jar.
How many people are mixed-handed?
Interestingly, quite a few people are mixed-handed when data from psychological studies that include three categories for handedness are analyzed. The afore-mentioned large-scale meta-analysis of handedness studies also included a separate analysis that included only studies with three handedness categories. In those, the percentage of mixed-handedness was 9.33 percent (Papadatou-Pastou, and co-workers, 2020). Considering that the number of people on earth is roughly 8.3 million, this means that approximately 774 million people worldwide are mixed-handed.
How does someone know whether they are mixed-handed?
As most parents are not aware that there is a third handedness category, the vast majority of mixed-handed people were told by their parents that they were either left-handed or right-handed. Thus, it is likely that there are many people who are mixed-handed but not aware of it! To find out whether someone is mixed-, left-, or right-handed, the person would generally need to fill out a hand preference questionnaire about which hand they prefer using for different activities.
For example, if you want to know whether you are mixed-handed, think of different activities that you can perform with your hands, such as cutting, writing, drawing, and so on. If you only prefer one hand for all activities, you are likely left-handed or right-handed. If you tend to use the left hand for some activities and the right hand for others, chances are high that you are mixed-handed.