I know this because it's discussed on the first day of classes at DLI as for why you need to learn to speak the language not just hear and read it. Because my job title was voice intercept operator.
Pre-1976 the Defense Language Institute did not score or rate students speaking ability because only reading and listening were considered mission critical skills.
Post-1976 it was considered a mission critical skill because it had such a dramatic effect on students final listening and reading abilities during the culminating Defense Language Proficiency Test (DLPT). Subsequently an Oral Proficiency Interview (OPI) was developed and is considered an integral part of the DLPT.
Note: This was all direct, primary research done at the Defense Language Institute with thousands of participants annually, so it was direct cause and effect experiment.
It's hard to find the specific research results summarizing this, but here's one example from that time period. https://psycnet.apa.org/record/1975-10458-001