Recently, there have been a number of news articles about how lead and smoking during pregnancy may lead to ADHD. Here is one such article. While these findings are important, I want to put them into context.
The causes of ADHD have been studied extensively. While there is always room for further understanding and research, we have great knowledge now. An excellent new book on the causes of ADHD was recently published, and is the most comprehensive review of the research to date. It is called: What causes ADHD? Understanding What Goes Wrong and Why By Dr. Nigg
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The facts are that about 80% of the cause of ADHD is genetic, and 20% is acquired.
Genetic ADHD
Research has shown that ADHD clearly runs in families. Twin studies, family studies and now genetic research have clearly demonstrated the genetic link of ADHD. Geneticists have identified approximately 5 ‘candidate genes’ which can be an issue in ADHD.
When looking at the ‘heritability’ of a disorder, researchers look at what percentage of the cause of a disorder is due to genetics.
Here is a comparison of heritability of different medical conditions:
The heritability of breast cancer: 30%
The heritability of asthma: 40%
The heritability of height: 85%
The hertitability of ADHD: 80%
So, ADHD clearly runs in families, and genetics account for about 80% of the known cause.
What accounts for the other 20%?
Acquired Causes of ADHD
After we account for the genetic causes of ADHD, there are several acquired causes.
The acquired causes can be broken down into Prenatal (i.e. before birth), and Postnatal (after birth).
Prenatal Causes of ADHD:
Prenatal causes of ADHD include the events that happen while the mother is pregnant. They include:
- Maternal smoking during pregnancy (2.5 x risk)
- Maternal drinking alcohol during pregnancy (same risk)
- Prematurity of birth (especially if brain bleeds - 45% have ADHD)
- Overall increase in pregnancy complications
Postnatal causes of ADHD:
Postnatal causes of ADHD are the events that occur after birth. They include:
- Head injury
- Brain hypoxia (i.e. lowered oxygen to the brain)
- Brain tumor
- Brain infection
- Lead poisoning in preschool years (0-3 yrs old)
- Survival from Acute Lymphoblastic Leukemia (’ALL’ - the treatments cause ADHD)
Where does the quoted study fall into this information?
As is always the case in science, new studies help to replicate old findings, and carry the information forward. This study reiterates the risks of smoking and lead exposure, and moves our understanding forward in this regard.
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