Now all my adolescent difficulties begin to make sense. Smart girls may be harder to get into bed because they rightly avoid or postpone emotional entanglements proposed by immature but eager boys using catchphrases like: “Have you read Franz Kafka?” It seems, retrospectively, that this ploy did not work for me, not because of the rather gloomy tone of this particular author, nor because of other personal deficiencies, but because of a general hesitancy about sex on the part of bright young women. A useful finding.
Into these passionate matters jump Rodgers and Garrison, with a very crafty use of a rich and historical linkage between dataset to tease out between family effects (used in the previously published literature) and within family effects.
They say: Using the NLSY79 data and the NLSY-Children/Young Adult data, we linked the NLSYC/YA children to the NLSY79 mothers.
That’s right. We have a sample who have been IQ tested in adolescence and who now have children who take part in this study.
We measured maternal IQ using the NLSQT, and children IQ using PPVT, PIATMath, PIAT-Reading Recognition, PIAT-Reading Comprehension, and Digit Span. We measured both maternal and child age at first intercourse (AFI) using self report from the respondents themselves. We fit these measures using Kenny’s (2001) reciprocal standard dyad model. This model supported analyses treating the data as only between-family data (as in past studies), and also allowed us to use within-family comparisons. These included two forms, first a comparison of offspring of mothers in relation to maternal IQ , then a comparison of offspring themselves in terms of offspring IQ. When we evaluate the relationship between both maternal and child intelligence, using a between-family design, we replicate the earlier results.
So far, so good, a replication.
When we use a within-family design, however, and compare offspring of sisters, and then compare the offspring themselves, the relationship between both maternal and child intelligence and age at first intercourse virtually disappears. The finding is robust across gender. These results suggest that the cause of the intelligence-AFI link is not intelligence per se, but rather differences between families (such as parental education, SES, etc.) that correlate with family-level (but not individual-level) intelligence.
The more we use linked data across the generations the better we will understand the interplay of general and family specific factors