Part 2: David’s Physical Condition
At his trial, David’s defence argued that his behaviour had not
been under his control. Shortly before his behaviour changed,
David had accidentally been exposed to a high concentration
of an organophosphate pesticide called carbaryl. Carbaryl is a
substance that can have an effect on the nervous system. In
fact, it behaves similarly to many nerve gasses (chemical weap-
ons that exert their effect by impairing the functioning of the
nervous system).
Carbaryl works by affecting the way that brain cells communi-
cate with each other. This communication occurs at synapses,
little gaps at the junctions of nerve cells. Neurones (nerve cells)
release chemicals to send signals across the gap. These chem-
icals are called neurotransmitters. After it has been released,
the neurotransmitter is broken down to prevent it from stimu-
lating the post-synaptic neurone for any longer than necessary.
Carbaryl prevents the breakdown of a neurotransmitter called
acetylcholine (ACh). Exposure to high doses of carbaryl causes
ACh to build up in the brain and other parts of the nervous system.
One of the many parts of the brain that uses ACh is a structure called the hypothalamus. This structure is im-
portant in a huge range of behaviours including eating, drinking, sex, and aggression. Electrical stimulation of
specific parts of the hypothalamus can cause a rat to show signs of rage and attack behaviour. David’s defence argued that the carbaryl to which he had been exposed had
affected the functioning of his hypothalamus. A small provocation would have
started the parts of the hypothalamus associated with attack behaviours to
start firing. However, because of the carbaryl, they would not have been able to
stop, leading to uncontrolled rage and aggression.
Discussion Questions:
• Based on the evidence provided, do you think David is morally responsible
for his crime?
• What possible environmental influences may have incited David to act as he did?
• Why do you think the jury believed the prosecution in spite of the defence’s
argument?
• How does this case affect your views on the nature versus nurture debate?