average intelligence of humans was going down because the weakest and stupidest don't die off early as was the case in earlier times

Articles proposing this thesis have been appearing ever since 1880, when psychologists began studying the inheritance of intelligence. To the best of my knowledge, none of them hold water. The only widespread genetic change in the last 20,000 years has been the dramatic spread of the gene for lactose tolerance, which powerfully improves the survivability of infants.

Objective studies since 1900 demonstrate quite convincingly that average IQ has actually been rising, not falling. It has a lot to do with changes in nutrition, exposure to lead, and neonatal care. This article discusses the phenomenon in considerable detail:

en.wikipedia.org/wiki/Flynn_effect

I hope this helps...

Loren