The Vandenberg and Kuse paper[1] followed up on this using a test that was easier to administer to large groups. In that study, the average male score was the same as the 85th percentile of female scores.
"Significant" describes this difference much more accurately than "slight."
The male versus female is amazing, but what about that drop in ability from age 29 to 33. I know one of the video game FPS coaches said that after you turn 30, one is out of the game, but look at that drop. There should be studies to figure out what causes that; it is impacting male and female.
The drop is probably multiple factors, including aging. The study was done in 1975. Someone 28 years old would have been born in 1947. A 33 year-old would have been born in 1942 –the darkest days of WWII. Prenatal nutrition was likely an issue then. Ditto for those born earlier, as they had to deal with the great depression.
Also, the sample size in the first study was rather small, as they were just trying to answer the question, "Does the time it takes to mentally rotate an object depend on the angle of rotation?" (Answer: yes.)
