The average of the population? (In this case, the average SSC reader.) If you have a big enough sample, you can ask if the average only child scores higher "Op

The average of the population? (In this case, the average SSC reader.) If you have a big enough sample, you can ask if the average only child scores higher "Openness" than the average reader, or if their self-reported IQ is higher. That's a meaningful comparison I believe.