Experiments suggest that it’s not until a child is seven that she has the mental capacity to create the kind of stone tools Homo erectus created. In other words, Homo erectus may have had the intelligence of a Western seven-year-old. On the WISC-R IQ test, an incipient adult (age 16.9) who performs like a seven-year-old on the spatial construction subtest scores lower than 99.5% of biologically normal members of his generation. In other words, an IQ of about 60.
But we should keep in mind that the research on seven-year-old tool making ability was published in 1979 (35 years ago). Probably because of better nutrition, spatial ability has been improving, even in recent decades, though only by about 0.2 points a year. So Homo erectus probably had an IQ around 53 on modern Western norms.
Brain size of Homo erectus
According to research cited by scholar Richard Lynn, Homo erectus emerged 1.7 million years ago with an average brain size of 885 cc and by 200,000 years ago, their brains had increased to 1,186 cc. How does this compare to modern Western brains? Data from scholar J.P. Rushton shows that Caucasian enlisted men in the U.S. army have a mean cranial capacity of 1468 (Standard Deviation (SD) = 91) and for Caucasian women the mean and SD are 1284 and 90 respectively. From here it can be deduced that a sex-combined sample (that was 50% men, 50% women) would have a mean and SD of 1376 and 137 respectively.
But because this data is from 1989 and U.S. brain size has probably increased by about 2 cc per year because of nutrition, we should assume the young adult Western brains today have a mean of 1426 cc (SD 137). So near the end of their run, when Homo erectus averaged 1,186 cc, they were 1.75 SD smaller brained than modern Western Caucasians. Since brain size has a true general intelligence loading of 0.53, we divide 1.75 SD by 0.53 to see that Homo erectus had 3.3 SD less general intelligence than modern Westerners have. However since IQ tests only correlate 0.85 with general intelligence (0.94 if we adjust for a 0.9 reliability of IQ tests), we multiply 3.3 SD by 0.94 to see that Homo erectus would have had an IQ 3.1 SD lower than modern Western Caucasians, or roughly IQ 53.
Based on the tools they made, Homo erectus had a historiometric IQ of 53, and based on a brain size validity generalization, they also had an IQ of 53. No exact solution is possible of course, but this is an impressive level of agreement.