It's two standard deviations and yes, there is frustration in dealing with people you cannot effectively communicate with. One standard deviation allows communication more or less effectively, i.e within the military you want the nco/officers to be smarter, and the grunts to be lower intelligence while still retaining effective communication. A second observation can be made with the US special forces, which tend to require an IQ of 140 or higher, 130 or higher with a doctors note approved by the military "councils". This is due to needing everyone to be on a relatively equal footing due to working in smaller teams while needing to retain effective combat roles. If someone with 120IQ was allowed they no doubt would fuck up the rest of the platoon somehow.
