A sample of singleton children (n = 33,437) was drawn from the US Collaborative Perinatal Project. The outcome measures were assessed at 8 mo, 4 y, and 7 y (Bayley scales, Stanford Binet Intelligence Scale, Graham-Ernhart Block Sort Test, Wechsler Intelligence Scale for Children, Wide Range Achievement Test). The main analyses examined the relationship between neurocognitive measures and paternal or maternal age when adjusted for potential confounding factors. Advanced paternal age showed significant associations with poorer scores on all of the neurocognitive measures apart from the Bayley Motor score. The findings were broadly consistent in direction and effect size at all three ages. In contrast, advanced maternal age was generally associated with better scores on these same measures.
Below the fold I've reformatted some numbers from a model the authors offered in the paper:
|Paternal Age 20||Paternal Age 50|
|Bayley Mental||79.8 ( 79.6-80.0)||79.2 (78.8-79.6)|
|Bayley Motor||33.9 (33.7-34.1)||33.2 (32.8-33.6)|
|Stanford Binet||108.8 (108.0-109.5)||106.6 (105.2-107.9)|
|Graham Ernart||38.0 (37.7-38.4)||36.9 (36.2-37.5)|
|WISC Full Scale IQ||104.9 (104.2-105.6)||103.0 (101.8-104.1)|
|WRAT Reading||107.2 (106.4-108.1)||104.7 (103.2-106.3)|
|95% C.I. in parantheses|
|Adjusted for sex of offspring, gestational age, other parent's age, mother's race, socioeconomic index, marital status, and maternal and paternal psychiatric illness|
As you can see, controlling for many factors older fathers seem to produce duller offspring. A few points of difference in the mean can have large effects on the tails, so this is not trivial. The biological rational for this is laid out in the paper, older fathers are likely to pass on more mutations to their offspring because sperm replicates enough that errors are likely to slip past DNA repair mechanisms. Older fathers within a population increases genetic variation by increasing genetic load (therefore, the contention that de novo mutations tend to come through the paternal line), but may also decrease mean fitness. I'm not too interested in the intelligence tests for the super-young, but the WISC and WRAT for 7 year olds is notable (though it would be critical to follow up into adulthood as the effects of paternal age might wear off).
Now, some data from the GSS, here is the mean age when the first child is born to categories of individuals who have a particular score on a vocabulary test. In short, the stupid people are on the left and the smart on the right (0 out of 10 vs. 10 out of 10).
As you can see the smarter you are the older you are when your first child is born. I doubt this surprises anyone. From the paper:
With respect to the mechanism of action underpinning these findings, several hypotheses warrant further scrutiny. While twin studies have demonstrated that cognitive ability and brain structure are heritable...studies based on sibships within the CPP have also confirmed that socioeconomic factors play a role in mediating the heritable aspects of intelligence...With respect to paternal age, a broad range of socioeconomic factors improve with increasing age, thus most commentators believe that the offspring of older parents would have better access to health and educational services compared to the offspring of younger parents (who tend to have lower education and poorer income)...For example, Fergusson and Lynsky...found that offspring of younger mothers tended to be born into relatively poorly educated and socially disadvantaged families. These authors commented that children born to young mothers were exposed to less nurturing and more changeable home environments. One would expect that such mechanisms would also operate with respect to paternal age. Clearly, our findings linking APA with impaired neurocognitive development cannot be readily explained by these social mechanisms.
There's an alternative model here: those who have children young are simply less intelligent and more impulsive on average, and these traits are heritable. But ultimately this still doesn't refute the primary finding of this paper, that older fathers produce duller children than we would expect them to, especially given the advantages that older fathers tend to have. Perhaps one might do a cost vs. benefit analysis about the storage of sperm by young men for utilization later on in life when they find that "special someone"?
Children of older fathers are also at a raised risk of schizophrenia and of autism, IIRC. Which makes sense w.r.t. de novo mutations.
That raises a point, though. They say in the paper that they controlled for parental psychiatric illness. But I doubt they'll have been able to fully control for that (a lot of mild cases go undiagnosed). Could that be an alternative explanation here?
"Perhaps one might do a cost vs. benefit analysis about the storage of sperm by young men for utilization later on in life when they find that "special someone"?"
This pisses me off.
I spent $200 to speak with a Doctor at a fertility clinic about doing this exact thing (freezing my sperm, not doing a cost analysis). I'm 30, I'm going to have to have kids via in vitro anyway because I'm gay, and since I may not be ready for another 10 years or so, I was worried about passing on mutated genes. There's already serious mental illness in my family (bi-polar disorder and associated conditions) so I wanted to give my kids at least some fighting chance. And the stupid doctor poo-poohed the whole idea and charged me the $ anyway!
Well, I had my son at 45 and he plays piano, gets straight A's, speaks Mandarin and reads at a high school level. He is 8. So don't worry.
I skimmed over the paper and didn't yet read it thoroughly (so perhaps I missed it) but, did the study control for the IQ of the father? I.e., did they compare fathers in their 20s with the same IQ as fathers in the 50s? (And did they control for the IQs of the mothers?)
Perhaps it's just that there is a trend that father that have children in their 50s have, on average as a group, a lower IQ than fathers that have children in their 20s, on average as a group.
Pretty talented kid you got there. Your wife got any eggs she's looking to unload? :) Sorry, that was inappropriate...
Why would dumb men wait longer to have children?
Why would minor mean differences mean anything for the tail?
Assuming these are normal distributions and both groups have the same standard deviation, we'd have an equal number of 160 iq geniuses as 162 iq geniuses....which would mean what?
@Mark, you said...
Charles, Why would dumb men wait longer to have children
I'm not sure why it might be the case. But I don't think it's necessary to explain why it might be the case, to logically see that it is something that should be checked for so that a cogent argument can be made for the hypothesis of: old fathers giving more "bad" mutations that reduce the IQ of the offspring they father in their 50s.
 I could guess. But I don't think it is necessary. Just because I might not be creative enough to come up with a "good" counter-explanation doesn't mean one doesn't exist.
we'd have an equal number of 160 iq geniuses as 162 iq geniuses....which would mean what?
non-trivial diff. in the raw numbers in the interval between 140-160 is what i was thinking.
This article on Yahoo!, posted today, reminded me of your blog on here:
"Children of Older Men Suffer Lower IQ"
Why would dumb men wait longer to have children?
Because women don't want to date them?
Perhaps one might do a cost vs. benefit analysis about the storage of sperm by young men for utilization later on in life when they find that "special someone"?
Doesn't IVF result in higher mutation rate than the old fashioned method? So I don't think there's likely to be much benefit, if any.
Unless you use embryo analysis and selection in the future, in order to avoid the more mutated candidates. But that gets into much broader issues than just compensating for age-related decline!
Of course it's possible that dumber men wait longer to have children; I only asked why it might be so because I thought you might have a theory up your sleeve.
Isn't intelligence carried on the X-chromosome? http://jmg.bmj.com/cgi/pdf_extract/28/6/429
Or have recent studies disproven this theory?
Is there a chart that breaks out the results by gender?