A useful distinction within cognitive and brain sciences is that between fluid and crystallized ability. Although fluid ability is widely studied, crystalized ability, which draws on acquired, declarative semantic knowledge and the mental lexicon, has been less well studied. Partly this is due to the culture and language specificity of assessment methods. We developed and assessed the psychometric properties of a simple 42-item lexical decision task that could be used with Thai speakers to assess the breadth of their crystalized vocabulary knowledge. A large sample of responses from 662 Thai-speaking participants, collected online, was used to refine the scale through exploratory factor analysis, and establish its internal consistency. A smaller sample of 90 participants was interviewed to establish validity of the task as a measure of Thai vocabulary. Large positive correlations (i.e., > .3) were found between the Thai Lexical Decision Task and measures of verbal fluency, particularly in the first 30 seconds of responding, and with other measures of Thai language skill. Temporal stability of the scale was assessed in a subsample of 27 participants. This confirmed that the Thai Lexical Decision Task has little obvious practice effect and excellent test-retest reliability. We also observed the expected positive associations with age, educational level, and self-reported proficiency in the language, supporting the ability of the task to measure acquired, crystalized knowledge. This new task could be used to estimate cognitive ability in Thai adults and shows potential as a measure of premorbid cognitive function for use in neuropsychological assessments.

ities such as learning, reasoning, and decision-making ( Stanovich, 2009 ; Fletcher et al., 2018 ). This critique of the IQ tests leads to the rise of the broad theories of intelligence. The broad theories emphasize on the aspects of mental functioning associated with the vernacular use of the term intelligence including adaptation to the environment, display of wisdom, creativity, etc., irrespective of whether these aspects are amenable to measurement or not ( Gardner, 1993 ; Perkins et al., 1993 ; Ceci, 1996 ; Sternberg, 2018 ). They highlight aspects of mental functioning that shape human behavior that is otherwise largely ignored by the narrow theorists, including biological characteristics, psychological processes, and socio-cultural environs. However, the narrow theorists accuse the broad perspectives of deliberately expanding the conceptual scope and usage of the term intelligence to counter the elevated status of the IQ tests. They argue that the generous conceptualizations of broad theories strategically downplay the importance of the IQ tests by broadening the definition of intelligence to make them only a part of the larger whole ( Stanovich, 2009 ). Moreover, critics also highlight the fact that broad theories lack empirical grounding and exclusively rely on anecdotal evidence for support ( Stanovich et al., 2016 ). Overall, both the narrow and broad theories form the core of the longstanding debate regarding the nature of intelligence and its measurement. Nevertheless, the narrow approaches with the IQ tests at heart enjoy an excellent scientific and popular reputation in comparison to the broad theories. EG Boring’s (1923) famous assertion that “Intelligence is what IQ tests measure” reflects the status enjoyed by the IQ tests in the scientific community. The Interpretive Gap The use of IQ tests permeates many spheres of human activity ( Sternberg et al., 2001 ). The IQ tests are used to make decisions in a variety of contexts, including school and col