Strengthening and Weakening Arguments

Some of the most challenging LNAT questions ask you to identify what would strengthen or weaken an argument. These questions test your ability to understand the logical structure of an argument and to evaluate how additional evidence would affect the relationship between premises and conclusion.

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The Underlying Principle

Most arguments in LNAT passages are inductive — the premises make the conclusion probable, but do not guarantee it. This means there is always a gap between what the premises establish and what the conclusion claims. Strengthening and weakening questions ask you to identify information that would make this gap smaller (strengthening) or larger (weakening).

Key Insight: You are not being asked whether the additional information is true. You are being asked what effect it would have on the argument if it were true.

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What Strengthens an Argument?

An argument is strengthened by additional evidence or information that:

1. Supports a key premise — makes an existing premise more credible
2. Eliminates an alternative explanation — rules out other possible reasons for the evidence
3. Confirms a key assumption — provides evidence for something the argument takes for granted
4. Provides additional evidence for the conclusion — offers a new reason to accept the conclusion

Worked Example

Argument: "The introduction of speed cameras on the A47 has reduced road traffic accidents. In the year following installation, accidents on that stretch of road fell by 25%."

What would strengthen this argument?

Option	Strengthens?	Why
Accident rates on comparable roads without speed cameras did not fall during the same period	Yes	Eliminates the alternative explanation that the reduction was due to a general trend
The weather was unusually mild during the year following installation	No	This suggests an alternative explanation for the reduction (mild weather, not cameras)
Speed cameras on the A12 also led to a reduction in accidents	Yes	Provides additional analogical evidence supporting the causal claim
The cameras were installed at locations with the highest accident rates	No	This suggests "regression to the mean" — accidents may have fallen naturally regardless of cameras

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What Weakens an Argument?

An argument is weakened by additional evidence or information that:

1. Undermines a key premise — casts doubt on the truth or reliability of an existing premise
2. Provides an alternative explanation — offers a different reason for the evidence that does not support the conclusion
3. Challenges a key assumption — shows that something the argument takes for granted may be false
4. Provides a counterexample — offers evidence that directly contradicts the conclusion

Worked Example

Argument: "Organic food is healthier than conventionally produced food. A study found that organic vegetables contain higher levels of certain antioxidants."

What would weaken this argument?

Option	Weakens?	Why
Higher antioxidant levels do not necessarily translate into health benefits for consumers	Yes	Challenges the assumption that more antioxidants = healthier
The study was funded by an organic farming association	Yes	Undermines the credibility of the evidence (potential bias)
Organic vegetables are more expensive	No	This is about cost, not health — it is irrelevant to the argument
Conventionally produced vegetables contain higher levels of certain vitamins	Yes	Provides a counterpoint suggesting conventional food may have its own health advantages

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