How? If the majority of what determines an individuals IQ is genetic then how could it possibly have declined in the span of a few decades? Worst case scenario our predetermined genetic base line IQ is not improving as rapidly as it use to in the past.
However, I don't think there is strong enough evidence to jump to that conclusion. From what I understand a simple bad attitude can dramatically alter an individuals test score. No, I think the testing methodology and the scale of the testing would have to dramatically improve before a statement like: "People are getting dumber." can be taken seriously.
There are tons of socioeconomic factors to consider, but I know a lot more fairly average IQ couples with 4 kids instead of two, and I know a lot more smart people who are either childfree or single than parents.
The ability to use birth control properly requires some intelligence.
The needs/career ambitions of a knowledge work career sometimes push aside the desire for a family.
Not trying to be down on people who have lots of kids! There's just tons of factors, some less obvious.
Is it possible that you associate things that correlate with rejection of children with iq? Like ambitions and career - low ambition don't necessary imply stupid. May be that wish for children tamper ambition and career.
Current thoughts in research are that there are two major factors, each responsible for roughly half of the decline.
First is that intelligent people tend to obtain more education, and more educated people (especially more educated women) tend to have fewer children on average. This has quite obvious effects from evolutionary perspective.
The second, less obvious, is that evolution works by selecting more fit variants of genes out of the pool, and the pool is made of existing variants and random mutations that happen all the time. These random mutations are typically negative for fitness, so the evolution selects against them. However, humans these days are operating under extremely relaxed selection regime, and gene variants which would have had significant negative impact on fitness in the past, while have the exact same effect today, are no longer as detrimental for reproductive purposes, which is basically the only thing evolution cares about.
For a good reference, see Woodley of Menie & Figueredo, 2013.
Then why exactly was IQ ever on the rise in the first place? It sounds to me (based on the point your making) like the more advanced we get the lower the average IQ should be. Yet the article said prior to the 70's IQ was on the rise.
Other sources estimate the inflection point around 1850s, see Woodley of Menie & Figueredo, 2013 and their references. The reason was relaxation of Malthusian regime brought by the industrial revolution, and ensuing demographic transition.
Think about it this way: in pre-modern times, fertility rate was high, and women would bear many children, much more than 2 on average. Despite high infant mortality, still much more than 2 would survive childhood. Yet, the population has been growing extremely slowly. This implies that many people haven't had a chance to reproduce, and even among those who could, many of their children didn't, so on average people would have only around 2 descendants who would maintain the progeny line, despite relatively many births -- otherwise the population would have grown, and it didn't. We also have good evidence that it's more intelligent and richer people who enjoyed better reproductive success in pre-modern times, for example from the dying wills of medieval and pre-modern Englishmen. See Gregory Clark's "Farewell to Alms" for references, it's a great book by the way.
Also, the fact that IQ was on the rise was probably due the Flynn effect, which is confounding the issue. There are two effects going on: one is the Flynn effect, which is apparent growth in IQ scores without corresponding increase in what we tend to think of intelligence. The Flynn effect is unrelated to genetic influences on intelligence, it's unrelated to g (which is why it's not predictive of real life outcomes, since it's g that's predictive). At the same time, there was a sort of anti-Flynn effect that I'm talking about above, which is in fact genetic, and acts on g, so it is predictive of real world outcomes. The Flynn effect rise in IQ score was masking the drop in actual intelligence, and now that the Flynn gains are starting to plateu in developed countries, the second effect is starting to become more apparent.
It is but how long does it take? I do not believe it is as simple as saying:
Person A has an IQ of 102
Person B has an IQ of 104, therefore; the child will have an IQ of 103. There is no specific IQ gene and so I would imagine that it would take just as long from an evolutionary standpoint to decrease the average IQ as it would for us to loose our tails and stop living in tree's. It certainly wouldn't happen in a matter of 30 years. At least, I don't think it would... I could be wrong but I don't believe I am.
It certainly could happen in 30 years. We could make it happen much faster if we got a genocidal dictator who decided to kill smart people. Pol Pot may qualify.
Birth control is currently the primary natural selection criteria. Anything that can defeat birth control is extremely strongly favored by evolution. Evolution is quite rapid when the selection effects are strong, especially when also starting from a diverse population.
I think you are wrong but I am open to debate if your interested in posting the prerequisite, peer reviewed, case studies on selective breeding for IQ in humans.
I think most of the assumptions I have made a pretty reasonable. We are talking about the evolution of possibly thousands of different genes which could contribute to IQ. This can't be compared to altering the pod size of a pea plant, a rather simple genetic selection, which still takes many generations to take effect.
How much variation is explained by genetics versus environment depends on the range of genetics and the range of environments you're looking at. In the US you've got the the full range of normal h sapiens diversity but all children get to drink pasteurized milk, eat iodized salt, use indoor plumbing, and go to school. If you look at a country like India where that isn't true you'll see a much more pronounced impact of shared environment on IQ.
And it isn't that IQ is all genetic, it's that it's half genetic, half something we don't know how to measure, and very little shared or family environment.
However, I don't think there is strong enough evidence to jump to that conclusion. From what I understand a simple bad attitude can dramatically alter an individuals test score. No, I think the testing methodology and the scale of the testing would have to dramatically improve before a statement like: "People are getting dumber." can be taken seriously.