 The explanation for this correlation is that English and Spanish are descended from a common language, called Proto-Indo-European. Some, but not all, words in Proto-Indo-European began with a consonant we can label [P/F], which evolved into ‘P’ in Romance languages (including Spanish), and ‘F’ in Germanic languages (including English). (Note that it is the phonetic pronunciation, rather than the spelling that is of interest here; for example, many German words whose spelling begins with ‘V’ are counted for this purpose.) In the separate lineages leading from Proto-Indo-European to English and to Spanish, Proto-Indo-European roots were retained for some words, but replaced for others. The roots were retained often enough that the correlation can still be detected. Moreover, the two lineages evolved more or less independently after splitting from Proto-Indo-European. Note that the common cause is not the Proto-Indo-European language as a whole, and the effects are not English and Spanish. It would make little sense to assign probabilities to these languages (let alone a joint probability to English and Spanish). Rather, the common cause is an initial consonant [P/F] in Proto-Indo-European; the effects are initial ‘F’ in English and initial ‘P’ in Spanish. It makes sense to assign probabilities to these sounds, since we can count the frequency with which words in these languages start with these sounds. See  Figure 4.