What strategies are most effective at convincing people to consume fewer animal products and how effective are they? One way to answer this question is with a meta-analysis: an analysis of previous studies in which the best available research is combined to get an overall picture of what works and how well. A team of researchers did just this and found that, unfortunately, not much that’s been tried so far has been very successful.

The researchers focused on studies that used four different types of interventions:

1. Choice architecture nudges people to select more plant-based options — for instance, by making them the default option or placing them more prominently on menus.
2. Psychological appeals try to establish social norms around eating less meat. For example, one study looked at the effects of putting up signs in a restaurant advertising the popularity of their veggie options.
3. Persuasion involves providing materials related to animal welfare, the environment, or health intended to convince people to reduce their consumption of animal products.
4. Combinations of psychological appeals and persuasion typically employ a persuasive message alongside a norms-based appeal or an opportunity for people to pledge to reduce their meat consumption.

For their analysis, the researchers only included studies that used a randomized controlled trial, had at least 25 participants in each condition, and measured meat and animal product consumption outcomes at least one day after the intervention. They included every study they could find that met these criteria that was publicly available by December 2023.

This yielded a total of 41 studies comprising about 87,000 participants. The most common intervention (25 of the 41 studies, or 61%) was the use of persuasion, often through documentaries, leaflets, or op-eds.

Research On Diet Change Finds Very Small Effects

The researchers averaged the results of all the studies together to get an estimate of the effectiveness of all interventions, known as an effect size. The overall effect size was very small: just 0.07.

To put this into context, effect sizes can be restated in terms of “number needed to treat” — the average number of people you’d need to expose to an intervention in order for one person to have a “positive” outcome. For the effect size the researchers found, you’d need to try to convince roughly 50 people to reduce their consumption of animal products in order to succeed in getting one person to do so. Additionally, due to the limitations of some of the studies, it’s hard to say how much that person would actually cut back or for how long they’d stick with it.

Results for the different types of interventions varied but were similarly small. One exception was choice architecture, which produced a larger estimated effect size (0.21). However, this was based on only two studies, meaning that the estimate might be inflated.

Another exception was interventions that only aimed to reduce consumption of red or processed meat. These collectively produced an effect size more than three times larger (0.25) than for interventions aimed at reducing consumption of all animal products. But, as the researchers point out, interventions that only focus on red and processed meat — like guidelines for heart-healthy diets — may cause people to merely shift to eating more fish or chicken without reducing their total consumption of animals.

For these reasons, the researchers conclude that meaningfully reducing meat and animal product consumption is an “unsolved problem.”

Many Promising Strategies Haven’t Been Sufficiently Tested

However, one optimistic note from this study is that there are potentially many promising strategies left to explore. The researchers only found 41 studies that met their strict inclusion criteria, and most of them were published from 2020 onwards, meaning that this is a topic that’s only beginning to be studied in a rigorous way.

According to the researchers, some interventions that deserve further inquiry include:

• Spending time with farmed animals
• Adjusting the price of meat
• Showing popular TV shows and movies with veg*n or animal rights themes

For advocates, the lesson of this analysis is that getting people to change their diets to spare animals is difficult. But the available evidence on the question is incomplete and not all strategies have been tested.