First off, remember that IQ is a relative scale. The population mean is always 100. Therefore, you can gradually drag down a population's absolute intelligence while IQ remains constant, masking the reduction.
Clamping early: probably on its own at least 5 to 10 points.
Pitocin and vitamin K: Synergistically clamping at least 15 to 20 points.
Lack of breast feeding: 5 points.
Vaccination, including prenatal: HArd to put in terms of IQ because it's more of a behavior modification program, but I'll give it at least 10 points.
Circumcision: Behavior modification, not easy to correlate with IQ.
Fluoridation: 5 to 15 points. Depends on whether the level of halides is periodically spiked to strip off mineral layers in municipal lead pipes, the average intake of aluminum, other factors.
Chlorination: Further inhibition of iodine usage, base loss of at least 3 points. Scaled with iodine intake and other halides.
Brominated food: Scaled with other halides.
Irradiation: 10 points, effectively. Functional changes with memory and learning, fear processing, theory of mind, etc.
You can't easily conquer the adults, so you must act by infiltration, subversion, and perversion, to over a period of generations, take hold of each child born as raw material. Until he has been reduced, and remade in the image of a new world. The longer you defend these practices, the more you double down, the worse it's going to get. It's gotten far enough now that the eggs are pretty well in the scramble. Will you bury your head in the sand and die like an animal, or will you take hold of your own life?