After the second World War, my grandmother’s family scattered all over the world. My grandmother had already immigrated from Poland to the United States. Her youngest brother ended up in Vienna. Her three sisters, who didn’t look Jewish, managed to survive the war hiding in plain sight in Poland, then married conscripted French laborers and moved to France. Her two brothers immigrated to Brazil.
As a result, our occasional and joyous get-togethers with the extended family were a linguistic smörgåsbord featuring Polish, Yiddish, English, French, Portuguese, and Spanish. In particular, our Brazilian cousins spoke Portuguese to me and my father, and we spoke Spanish to them. At the time, I had the impression that our Brazilian cousins could understand our Spanish much better than we understood their Portuguese. This impression has stuck with me in later encounters with Portuguese speakers.
This isn’t just my own observation; ask any Spanish speaker, and you’re likely to hear the same thing. It’s also echoed in linguistic literature, most notably by eminent Spanish scholar Ralph Penny: “speakers of Spanish understand at least some of what is said in Portuguese, and the Portuguese speakers will understand a good deal more of what is said in Spanish” (Variation and Change in Spanish, p. 14).
I’ve only found one study that addresses this question systematically. Florida International University professor John B. Jensen had Portuguese and Spanish speakers (all from Latin America, and none expert at the other language) listen to passages in the other language, then answer comprehension questions in their own language. Comprehension in each direction hovered at around 50%. The Portuguese speakers were more successful than the Spanish speakers at interpreting what they heard, a difference that was slim yet statistically significant.
To find out why, I checked in with my great friend (and Portuguese expert) Bonnie Wasserman. She suggested that the main factor was the greater complexity of the Portuguese vowel system. Compared to Spanish’s economical five-vowel system, Portuguese has more core vowels and also a set of nasalized vowels. For example, where Spanish has a single vowel /o/, Portuguese has three: /o/ as in avô “grandfather” (this is closest to Spanish /o/), /ɔ/ as in avó “grandmother” (similar to the au of caught), and nasal /õ/ as in onda “wave” (the n is not pronounced). Spanish speakers can become confused if they fail to pick up on these subtle differences.
A second factor is rhythmic. While Spanish is more stacatto, with each word pronounced individually, Portuguese words are more connected. This makes it harder for Spanish speakers to pick out familiar words when heard in context.
[Update from 4 Aug: Here‘s a post about mutual intelligibility in English that includes a list of references about mutual intelligibility in general (search on “Subtirelu”)].