Results showed that mental rotation performance was affected by a significant interaction between sex, social and non-social traits, implying that social traits differently impacted on sex differences in mental rotation at high, middle, and low values of non-social traits. More precisely, when non-social traits were above the mean (+ 1 SD), no sex differences in mental rotation were found, whereas, below this value, sex differences depended on the levels of social traits, with men outperforming women in the case of middle-to-high social traits, and with a comparable performance, or even with women outperforming men, at lower social traits. Instead, no interaction was found between sex and traits when considering performance on the figure disembedding task, but only a small positive correlation between figure disembedding and social traits suggesting that higher social traits were generally related with a better figure disembedding performance. This last finding is consistent with Russell-Smith et al.’s [11] data, and also reveal that such a relationship, albeit weak, is independent from sex. Following recent evidence demonstrating that social and non-social autistic traits are dissociable even genetically [24], here we found that this was the case also when investigating the effects of social and non-social traits on a behavioral task as mental rotation. In this respect, mental rotation could be considered a behavioral proxy to non-social autistic traits, while is not true for figure disembedding.
The different effect social and non-social traits on mental rotation could involve the activation of different strategies to perform the task. Some authors suggest that mental rotation can be accomplished by two main strategies, global (holistic) and local (piecemeal) [40, 41]; the holistic strategy seems related to a better performance, whereas the piecemeal one to a poor performance [42]. Some have suggested that men on average may use a holistic strategy, whereas women on average may use a piecemeal strategy, although such differences have not been systematically confirmed [43, 44]. Recently, Stevenson and Nonack [15] studied eye fixations during mental rotation in women and men with low, medium and high autistic traits, as indexed by AQ total score. Results showed that fixations by the participants in the high autistic traits group varied by sex, while no sex differences were found for medium and low autistic traits, leaving unclear the possible mechanisms accounting for such a pattern of results. The present data underscore the importance to consider the extent to which persons differ in the degree of social and non-social autistic traits to clarify sex differences in mental rotation, because individual differences in non-social traits affect the way in which the individual deal with the task, possibly favouring activation of specific mental rotation strategies. Since a better performance may be related to a global strategy [42], one might suggest that higher non-social autistic traits would imply a stronger activation of the global approach both in women and man, thus flattening sex differences. Instead, at lower degrees of non-social traits, individual with higher social traits would tend to prefer a local strategy, being related to a lower accuracy. In the mental rotation literature, a global (or holistic) approach would imply that participants deal with the task by processing the elements of the to-be-rotated figures as integrated wholes, while the local approach would imply that the participants pay specific attention to the single components of the to-be-rotated figures through the entire mental rotation process [40, 45]. However, several scholars have pointed out that rather being divided into global vs. local, rotation strategies could be best divided based on their efficiency [46–48]. In particular, across several important studies, Just & Carpenter [47] demonstrated that the rotation process can be conceived as a piecemeal strategy since it entails a discrete, multi-step process in which the result of each step is monitored to determine if the intermediate result is approaching the final output. The authors also demonstrated that individuals performing better (high spatial subjects) are those who are able to keep track of the process, while individuals performing worse (low spatial subjects) are unable to keep track of the intermediate products of their partial rotations, so that their accuracy drops, especially at increasing task complexity. Just & Carpenter’s [47] perspective nicely fits the concept of hyper-systemizing according to which individuals with high degrees of systemizing are able to understand a system by identifying the key elements or variables of the system, then manipulating them systematically and checking the effect of such manipulation on the system through “if-and-then” rules [31, 32]. Thus, solving a problem through systemizing implies, as a first step of the process, detecting the crucial details of a complex whole (the system) then exploiting them to activate an “if-and-then”, multi-step process allowing system understanding and problem solution [31, 32, 49]. In the domain of mental rotation, people with higher systemizing or with high non-social autistic traits are prone to apply this multi-step process with high efficiency, thus performing better, as reported in the high spatial subjects, whereas people who perform low on mental rotation are less able to implement this multi-step process [47]. Indeed, our results showed that when non-social traits were below the mean there was the strongest sex difference with women being significantly low than men at higher social traits.
In summary, the present findings support the view that people with high non-social autistic traits show hyper-systemizing implying the capacity to step from details to the whole, also consistent the presence of talent in autistic people dealing with systemizing domains [31, 49]. In addition, high social autistic traits means the individual struggles with the capacity to move from details to the whole, leaving the individual stuck in the details. Since typical women, on average, are characterized by lower systemizing than men [30], this could account why men on average tend to outperform women on visuospatial tasks as mental rotation [12, 20]. Indeed, although men are generally higher than women in systemizing, not all men are high-systemizers and not all women are low-systemizers [13, 21, 31], thus when non-social traits are comparably high in women and men, a comparable mental rotation performance can be observed.
In conclusion, this study sheds light on the conflicting pattern of results in literature on the influence of autistic traits on mental rotation, paving the way for future investigations of this.