Beware: handedness bias in behavioral experiments

Article by Ben Howell

Photo by Александар Цветановић from Pexels

Photo by Александар Цветановић from Pexels

If you're not thinking about the effects of handedness in your experiments, you may be drawing false conclusions from the data collected.

Handedness can bias results in some experiments. By reducing the effect of handedness bias (or in some circumstances, by exploiting it), you can draw more accurate conclusions from the data you collect.

Throughout this article you'll learn practical methods for assessing handedness and for reducing the risk of bias introduced by handedness effects.


What is handedness?

Humans tend to be right-hand or left-hand dominant and generally display greater dexterity and more refined fine motor skills with their dominant hand. Where possible, humans also tend to prefer to use their dominant hand.

Similarly, laterality refers to the preference and dexterous dominance of one side of the body over the other (e.g., preferred foot for kicking a ball, forward foot on a bicycle, preferred eye for looking through a telescope, preferred ear for mobile phone). Laterality is not always consistent with handedness.


Why account for handedness?

Simon and Rudell (1967) showed that the location of stimuli directly influences response-selection due to a natural tendency to react towards the source of the stimulation. Their conclusion was that stimulus location produces interference in a response-selection process. Therefore where stimulus location and response-selection are congruent, rather than incongruent, accuracy is greater and reaction times are faster.

This is known as the Simon effect.

Run a live demonstration experiment to test the Simon effect:

  • Simon effect (desktop version, optimized for keyboard response)
  • Simon effect (phone version, optimized for portrait screen orientation and on-screen button response)

Exploiting the Simon effect in the design of machinery, tools, and human-computer interfaces leads to more intuitive and efficient solutions, and can significantly reduce human error.


Assessing handedness

The Edinburgh Handedness Inventory is a measurement for representing the handedness of a subject. The measurement is conducted by way of a short questionnaire. Responses given are used to calculate the Laterality Quotient of the subject.

The Edinburgh Handedness Inventory, originally devised by Oldfield (1971), has since been simplified by Veale (2013). The Edinburgh Handedness Inventory - Short Form (Veale, 2013) can be seen below (Figure 1).

Figure 1. Edinburgh Handedness Inventory - Short Form (Veale, 2013; Psychstudio, 2020)

Figure 1. Edinburgh Handedness Inventory - Short Form (Veale, 2013; Psychstudio, 2020)

Scoring the questionnaire is simply a matter of adding the answers together from each of the questions according to the table below

Scoring the Edinburgh Handedness Inventory questionnaire
AnswerScore
Always right100
Usually right50
Both equally0
Usually left-50
Always left-100

Finally, to calculate our Laterality Quotient, we take the questionnaire total and divide by 4.

Handedness as defined by the Edinburgh Handedness Inventory
HandednessLaterality Quotient
Left handed-100 to -61
Mixed handed-60 to 60
Right handed60 to 100


Neutralizing the effect of handedness in behavioral experiments

In general, when your experiments have large enough sample sizes for statistical power and when response allocation is randomized, any bias introduced by handedness will be washed out of your results. Randomizing response allocation involves randomizing button placement, randomizing key/value assignments, etc.

Randomizing response button order in Psychstudio

When two or more buttons are present, you can randomize their layout order from inside the editor (Figure 2) by selecting

For example, in Figure 2, two buttons are assigned:

  • False leftmost placement
  • True rightmost placement

When randomization is enabled, False will be randomly allocated to either leftmost or rightmost placement and True will be randomly allocated to one of the positions that remain (in this case there will be only one). Randomization remains consistent across all trials.

Figure 2. Randomize button order example (Psychstudio, 2020)

Figure 2. Randomize button order example (Psychstudio, 2020)

Cloning response buttons in Psychstudio

In order to maintain the same button order across multiple trial procedures and/or multiple trial blocks, you can clone a buttons response on the trials timeline to keep a consistent layout across all buttons. So for example, if a randomized button layout produced the layout as seen in Figure 2 above (False on the left, True on the right) every Response - buttons in every trial block and trial procedure in the entire experiment will contain the same layout (False left, True right) when using a clone.

  1. click New item
  2. select the Response - buttons type (Figure 3)
  3. select the Clone (from a different procedure) creation option (Figure 3)
  4. select the buttons response item you wish to clone (Figure 4)

Figure 3. Cloning a buttons response (Psychstudio, 2020)

Figure 3. Cloning a buttons response (Psychstudio, 2020)

Figure 4. Select the buttons response you previously created (Psychstudio, 2020)

Figure 4. Select a buttons response you've previously created (Psychstudio, 2020)

Randomizing key values in Psychstudio

When two or more keys are present, you can randomize their key/value assignments from inside the editor (Figure 5) by selecting

For example, in Figure 5, two keys are assigned:

  • f = left
  • j = right

When randomization is enabled, f will be randomly allocated to either left or right and j will be randomly allocated to one of the values that remain (in this case there will be only one). Randomization remains consistent across all trials.

Figure 5. Randomize key/value assignment example (Psychstudio, 2020)

Figure 5. Randomize key/value assignment example (Psychstudio, 2020)

Cloning response keyboard in Psychstudio

In order to maintain the same key/value assignments across multiple trial procedures and/or multiple trial blocks, you can clone a keyboard response on the trials timeline to keep consistent key/value assignments across all keyboard responses. So for example, if a randomized key/value assignment produced the assingments as (f = left, j = right) every Response - keyboard in every trial block and trial procedure in the entire experiment will contain the same key/value assignments (f = left, j = right) when using a clone.

  1. click New item
  2. select the Response - keyboard type (Figure 6)
  3. select the Clone (from a different procedure) creation option (Figure 6)
  4. select the keyboard response item you wish to clone (Figure 6)

Figure 6. Cloning a keyboard response (Psychstudio, 2020)

Figure 6. Cloning a keyboard response (Psychstudio, 2020)

References

  1. Oldfield, R. C. (1971). The assessment and analysis of handedness: The Edinburgh inventory. Neuropsychologia, 9(3), 97–113. doi: 10.1016/0028-3932(71)90067-4
  2. Simon, J., & Rudell, A. (1967). Auditory S-R compatibility: The effect of an irrelevant cue on information processing. Journal of Applied Psychology, 51(3), 300-304. doi: 10.1037/h0020586
  3. Veale, J. (2013). Edinburgh Handedness Inventory – Short Form: A revised version based on confirmatory factor analysis. Laterality: Asymmetries of body, brain and cognition, 19(2), 164-177. doi: 10.1080/1357650x.2013.783045

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Ben Howell
Ben Howell
Founder, Psychstudio