Fast performance on simple cognitive tasks such as copying digits and symbols, locating identical pictures, or rapidly naming objects correlates with other cognitive skills such as logical reasoning, vocabulary, and memory [1, 2]. For this reason, psychomotor processing speed (PS) is considered to be one of the factors of intelligence. For example, in the Cattell-Horn-Carroll  theory, processing speed is one of the nine major broad abilities, and the predominant intelligence test, the Wechsler Adult Intelligence Scale , includes the PS index as one of the four major subscales.
While it is widely agreed that processing speed is an important element of cognition, there is no consensus on the exact nature of this construct. One of the major questions is whether speed is a unitary ability or instead is a collection of several cognitive processes [5, 6]. Based on the observation that performance in reaction time tests is associated with a wide variety of more complex mental skills, Jensen  has suggested that mental speed is a basic process that underlies general cognitive ability. However, studies of age-related changes in cognition suggest that a variety of neural systems affect processing speed [8, 9].
Another problematic feature of PS is that there seem to be differences across sex and nationality. For example, on the Wechsler tests (WAIS, WAIS-R, WAIS III and WAIS IV) [1, 10,11,12], there is a small difference in favor of males on the nonverbal, verbal, and working memory subtests, while females outperform males on the PS tests. In the latest version, WAIS IV, the index scores for males and females were 101.8 and 98.4, respectively, on the Perceptual Reasoning (PRI), Verbal Comprehension (VCI) and Working Memory (WMI) composite indexes and 97.7 and 102.1 on the Processing Speed index (PSI). There are also significant cross-national differences in IQ test profiles. In a recent study  the mean PRI scores for Finnish, Scandinavian, German and French WAIS IV standardization samples varied from 104 to 109 while the PSI scores varied from 96 to 101 when scored using U.S. norms (U.S. mean = 100 for all indexes). Thus, the WAIS factor model involving the four index scores is not perfectly consistent across sex and nationality.
Several factors have been suggested to underlie the differences in the PRI/PSI ratio between men and women, and across nations . The size of hands and finger thickness has been shown to affect tasks involving dexterity [15, 16]. In fine motor tasks, persons with narrow digits are at an advantage , and females have smaller hands than men do. Recent studies show that populations from cold regions have relatively shorter and wider fingers, a trait that is negatively correlated with dexterity [18, 19]. This may also explain some of the cross-national differences observed on PS tests. Another factor that may potentially affect processing speed is reading and writing skills, especially on tests that use digits and letters as stimuli. Studies such as the OECD PISA  based on very large samples (n > 100,000) find that females have better reading and writing skills than males. Lynn and Mikk  estimated the male/female gap to be d = 0.041 in the PISA 2010 study, and women are faster in handwriting in languages with very different orthographic systems, such as English , Chinese , and Japanese . The female superiority has been explained by the fact that females study more and do more homework . Increases in PS over generations  also suggests that practice effects based on rising educational levels may underlie observed differences in PS scores.
Differences in PS observed between nations and across gender suggest that there are several factors that affect PS and that these factors may act in different directions. For example, Finnish students have consistently ranked among the top five nations in reading and writing skills in the PISA assessments between 2005 and 2018, while Finnish standardization samples have consistently had significantly lower PS scores on the different versions of the WAIS as compared to U.S. and European standardization samples [13, 26]. It has also been found that males perform better than females on some types of speed tests, but females perform better on others. Males tend to be faster on fairly simple tasks such as reaction time tests and finger tapping, while females outperform males in rapid naming and in tests involving clerical-type skills [27,28,29]. In addition to being related to general intelligence, dexterity and reading and writing skills, speed tests are also affected by test-taking attitudes. According to Erdodi et al. , a test profile with a PSI score significantly lower than scores on the reasoning indexes may be caused by poor test-taker effort.
When different factors that may counteract each other are studied in separate studies using separate samples, it is difficult to analyze the effects of each factor. In the present study, the effects of a wide selection of factors on processing speed were analyzed in a sample of outpatients. The dependent variable was processing speed, measured by performance on the WAIS IV processing speed subtests (Digit Symbol Coding, Symbol Search), and the independent predictors were age, sex, height, weight, education, type of medication taken, gross manual dexterity, fine motor speed, grip strength, depression, performance on tests of nonverbal reasoning, verbal reasoning, working memory, and logical verbal memory, and 15 different personality traits.
The goal of the study was to explore the effects of psychological and physiological factors on processing speed in a study design that provides the opportunity to control for the major confounding factors of education and gender. It was hypothesized that sex differences in processing speed may be explained by factors that (a) have a positive correlation with PS when sex is controlled for, and (b) show a female advantage in population samples.