Strengthening by Supporting Premises

The most intuitive way to strengthen an argument is to provide additional evidence for its existing premises. If an argument's premises are better supported, the foundation of the argument becomes more secure, and the conclusion becomes more probable. This lesson examines how this works in practice, with particular attention to the types of supporting evidence that the LNAT rewards you for recognising.

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How Supporting Premises Strengthens an Argument

An argument's strength depends partly on the credibility of its premises. If a premise rests on limited or questionable evidence, the argument is vulnerable. Providing additional evidence for that premise removes a weakness and makes the argument harder to attack.

Structure

Original argument	Additional evidence	Effect
Premise: "X is the case" (based on limited data)	New study confirms X with larger sample	The premise is now better supported, strengthening the overall argument
Premise: "Y causes Z" (based on one country's data)	Research from three additional countries shows Y causes Z	The causal claim is more robust
Premise: "Most experts agree that W" (no source cited)	A peer-reviewed survey of 500 experts confirms agreement on W	The claim of expert consensus is substantiated

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Types of Supporting Evidence

Different types of evidence provide different levels of support. Understanding these distinctions helps you evaluate LNAT answer options more effectively.

1. Empirical evidence

Direct observation or measurement that confirms a premise.

Argument: "Air pollution in cities is linked to respiratory disease." Strengthener: "A ten-year longitudinal study of 50,000 urban residents found that those living in areas with higher particulate matter levels had a 40% higher incidence of chronic respiratory conditions."

This strengthens by providing large-scale, specific, measured evidence for the premise.

2. Statistical evidence

Numerical data from representative samples.

Argument: "Young people are increasingly disengaged from traditional politics." Strengthener: "Voter turnout among 18–24-year-olds has fallen from 65% in 1992 to 47% in the most recent general election, and party membership in this age group has halved over the same period."

This strengthens by quantifying the claimed trend with specific, measurable indicators.

3. Corroborating evidence from independent sources

Evidence from a different source or method that reaches the same conclusion.

Argument: "Social media use is associated with increased anxiety in teenagers." Strengthener: "Three independent research groups — one in the UK, one in Canada, and one in Australia — have found similar associations using different methodologies and populations."

This strengthens because independent replication reduces the chance that the original finding was a fluke or an artefact of a particular method.

4. Expert testimony

Endorsement or confirmation from a recognised authority in the relevant field.

Argument: "The current sentencing guidelines for drug offences are disproportionate." Strengthener: "The Sentencing Council's own review concluded that sentences for drug possession are significantly longer than those for comparable offences in other categories."

This strengthens because the source is both relevant and authoritative — the Sentencing Council is the body responsible for guidelines.

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Worked Example 1

"Renewable energy sources such as wind and solar are now cheaper than fossil fuels in most markets. The government should therefore accelerate the transition to renewable energy, as this would reduce both carbon emissions and energy costs for consumers."

The argument's premises:

1. Renewables are cheaper than fossil fuels in most markets
2. Transitioning would reduce carbon emissions
3. Transitioning would reduce energy costs

Question: Which of the following, if true, would most strengthen this argument?

A. Several countries have signed international agreements committing to carbon reduction targets. B. A comprehensive analysis by the International Energy Agency found that in 85% of national markets, newly built solar and wind capacity is cheaper per megawatt-hour than newly built gas or coal capacity, and that consumer prices fell by an average of 12% in countries that increased renewable capacity by more than 30%. C. Public support for renewable energy has increased steadily over the past decade. D. A leading environmental charity has endorsed the proposal.

Answer: B. This directly supports two of the argument's key premises — that renewables are cheaper (with specific data on 85% of markets) and that the transition reduces consumer costs (12% reduction in countries that have transitioned). Options A and C are tangentially related but do not address the premises. Option D is an appeal to authority.

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Worked Example 2