The best objective evidence, from long term comparison of simple reaction times, suggests that there has been a considerable decline in general intelligence 'g' in Britain over the past 150-200 years - approximately one standard deviation (or 15 IQ points) decline compared with present norms .
The most plausible explanations are some combination of
1. Differential fertility between those with high and low intelligence - such that intelligence is inversely correlated with fertility, hence with reproductive success.
2. A generation-by-generation accumulation of deleterious (intelligence-reducing) genetic mutations due to the major decline in neonatal and childhood mortality rates.
Probably the same factors apply elsewhere in the world. Observations of manyfold worldwide population growth differentially greatest in Africa and the Middle East, plus the same patterning of fertility by intelligence, suggest that significant declines in general intelligence would be expected across most of the world.
If so, this would have many effects - one of which might be an increase in short-termism; in technical terms, an increase in delay discounting: that is the tendency to choose a smaller but immediate reward over a delayed but larger reward.
It has been found in numerous studies that there is a significant inverse correlation between intelligence and delay discounting - in other words the lower the intelligence, the greater the tendency to prefer more immediate but smaller rewards.
Delay discounting and intelligence: A meta-analysisNoah A. Shamosh , Jeremy R. Gray
Volume 36, Issue 4, July–August 2008, Pages 289–305
An increase in delay discounting is exactly what can be seen in several situations around the world - where stable long-term arrangements between different groups are breaking-down because the stronger group is no longer context to 'harvest' taxes and other benefits from the weaker groups over the long term; but instead the stronger group is expropriating the entire goods of the weaker group, now! - causing the weaker group to die or leave; or esle simply expelling/ killing them (e.g. 'ethnic, or religious, cleansing').
This is short-termist because it gains immediate benefit for some persons, but at the cost of rendering all parties worse-off in the longer term.
If the obvious increase in worldwide short-termism is indeed substantially due to a significant global decline in intelligence, then this counts as very bad news, because nothing much can be done about it.