This corpus-driven computational study addresses the question of why some verbs in some languages participate in the causative alternation while their counterparts in other languages do not. The results of this study suggest that the lexical property that underlies this variation is the probability of external causation. Alternating verbs are distributed on a scale of increasing probability for an external causer to occur. The probability of external causation can be empirically assessed in two ways, among others: first, by observing the typological distribution of causative and anticausative morphological markings across a wide range of languages; second, through the frequency distribution of causative and anticausative uses of the alternating verbs in a corpus of a single language. Our study reveals that these two measures are correlated. Moreover, we demonstrate that the corpus-based measure is applicable to a wide range of verbs. Extending the corpus-based investigation comparatively across two languages, English and German, we find that the frequencies of crosslinguistic realizations of lexical causatives are modulated by the probability of external causation, an underlying parameter assigned to verb types. Finally, we propose a probabilistic graphical model that clusters verbs based on the relation between the crosslinguistic distribution of their causative and anticausative realizations and the probability of external causation.