Background
Despite the importance of lifelong learning as a key to individual and societal prosperity, we know little about adult motivation to engage in learning across the lifespan. Building on educational psychological approaches, this article presents a measure of Motivation-to-Learn using four items from the background questionnaire of the Programme for the International Assessment of Adult Competencies (PIAAC).
Methods
We used multiple-group confirmatory factor analyses for ordered categorical data to investigate the scale's dimensionality and measurement invariance across countries. Regression analyses were used to investigate the scale's criterion validity.

Results
Results show that the proposed four-item scale fits the data considerably better than the original six-item scale labeled Readiness-to-Learn. Further analyses support the scale’s configural, metric (weak) and partial scalar (strong) measurement invariance across 21 countries. As expected, Motivation-to-Learn has significant relations to the working population’s engagement in learning in terms of participation in non-formal education over the last 12 months. These relations remain relevant after taking literacy as an indicator of level of education into account.

Conclusion
The Motivation-to-Learn scale presented here may be used to indicate adult motivation in cross-country comparisons. The potential of using the scale in future PIAAC analyses and research on adult learning is discussed.