APA (7th ed.) Citation
Humboldt, W., Buch, G. C., & Raven, F. A. Linguistic variability: Intellectual development.
Chicago Style (17th ed.) Citation
Humboldt, Wilhelm, George C. Buch, and Frithjof A. Raven. Linguistic Variability: Intellectual Development.
MLA (9th ed.) Citation
Humboldt, Wilhelm, et al. Linguistic Variability: Intellectual Development.
Warning: These citations may not always be 100% accurate.