Are birth order effects on intelligence really Flynn Effects? Reinterpreting Belmont and Marolla 40 years later

I reinterpret a forty-year-old finding by Belmont and Marolla (1973), who believed their Dutch IQ patterns were caused by within-family processes related to birth order. However, their inferred relation was almost certainly caused by differences between families — in parental IQ, maternal education, and/or dozens of other processes. I show that the Flynn Effect (which emerges from and is likely caused by combinations of such between-family processes) can theoretically account for the Belmont and Marolla patterns. I then draw on past research and additional analysis to show that the Flynn Effect was actually occurring in The Netherlands at the correct time and magnitude to explain the Belmont and Marolla patterns.