An Incomplete List of Things that Have Been Postulated, In Earnest, as Possible Causes of Autism

(Note: Not all of these ideas are crank hypotheses; some of them are, a few were widely accepted in their day but are now regarded as crank notions, many are just people throwing things out there on the strength of statistical correlations, some have a fairly impressive body of evidence supporting them, and some are ones that I consider crankish but not everyone does -- the Extreme Male Brain theory being an example of this latter category. I do not use my "bizarre autism hypotheses" tag exclusively for bad science; just everything that strikes me as odd.)

In no particular order:

• Vaccines
• Thimerosal in vaccines
• MMR vaccines
• Too many vaccines weakening a child's immune system
• Aluminum in vaccines
• Human fetal tissue in vaccines
• Air Pollution
• Excessive Androgen Exposure During Fetal Development
• Genetic Mutation
• Assortative Mating of Brainy, Socially Awkward Types
• The Internet
• Overactivity of the Endogenous Opioid System (PDF)
• Meat Eating
• Vitamin D Deficiency (PDF)
• Your Mom
• Your mom's age (PDF)
• Your mom's failure to bond with you when you were a baby
• Your mom was sick while she was pregnant with you
• Your mom was taking antidepressants while she was pregnant with you
• Your mom was taking anticonvulsants while she was pregnant with you
• Your mom had you via C-section
• Your mom conceived you using in vitro fertilization
• Your mom had you too soon after she had your next-oldest sibling
• Your mom gained too much weight during pregnancy
• Your mom did not have good nutrition while she was pregnant with you (1,2)
• Your mom smoked while she was pregnant with you
• Your mom had high blood pressure while she was pregnant with you
• Your mom had diabetes while she was pregnant with you
• Your mom had pre-eclampsia while she was pregnant with you
• Your mom gave birth to you prematurely
• Your mom has ADHD
• Your mom was under a lot of stress while she was pregnant with you
• Your mom didn't breastfeed you
• Your mom used hormonal contraceptives at some point in her life
• Your mom had an ultrasound while she was pregnant with you
• Your mom took too many folic acid supplements while she was pregnant with you
• Autoimmune Disease
• Yeast
• Circumcision
• Your Dad's Age
• Too Many Connections Between Neurons
• Not Enough Long-Range Connections Between Neurons
• Too Many Short-Range Connections Between Neurons
• Not Enough Long-Range Connections and Too Many Short-Range Connections Between Neurons
• Not Enough Short-Range Connections Between Neurons
• Undergrowth of the Corpus Callosum (PDF)
• Having Autistic Older Siblings
• Electromagnetic Radiation
• Pesticides
• Underactivity of Inhibitory Neurons Coupled with Overactivity of Excitatory Neurons (PDF)
• Perfumes
• Not Enough Sodium in the Blood
• Dysfunction of the Mirror Neurons (PDF)
• Food Allergies
• Too Much TV
• Excessive Intestinal Permeability
• Early Injury to the Cerebellum
• Early Brain Overgrowth
• Excessive Reactivity and Plasticity of Neural Microcircuits in Specific Areas of the Neocortex and in the Amygdala
• Dysfunction of the Amygdala
• Autistic People Are Human-Neanderthal Hybrids (added 1/24)
• Defective Mitochondria (added 1/24)
• Malfunction of the Methionine Cycle and Transsulfuration Metabolic Pathway, which Leads to Excessive Oxidative Stress (added 1/24)
• Malfunction of the Nitric Oxide Signaling Pathway (added 1/24)
• Trampolines, Bouncy Castles, Stationary Bicycles and Treadmills (added 2/9)
• Drinking Milk (added 4/15)

This is an incomplete list, so feel free to add anything you've heard that I have not listed here in the comments!