sword and shield hypothesis
The Sword and Shield Hypothesis proposes that systematic asymmetries in hand actions drive the laterality of affective motivation in the brain. The dominant hand is used disproportionately for approach actions (wielding a sword to attack), while the nondominant hand is used for avoidance actions (raising a shield to defend).
Because the right hand is controlled by the left motor cortex in right-handers, this could explain why approach motivation is left-lateralized. Previous work found that approach motivation is associated with left-hemisphere activity in right-handers but right-hemisphere activity in left-handers.
the experiment
I worked on a replication study of Berkman and Lieberman (2010), which found that approach motivation, rather than valence, drove left lateralization in prefrontal cortex. Instead of fMRI, we ran an online reaction time experiment.
Participants read about a fictional group called the Nochmani who eat insects and sweet things but avoid meat and fungus. Then they saw images of items in each category and indicated whether they would eat or not eat each item if they were Nochmani. This orthogonalizes motivation and valence: positive-approach, negative-approach, positive-avoidance, negative-avoidance.