Reducing animal suffering is one of the main focus areas of the Effective Altruism movement.  Currently, the dominant approach to reducing animal suffering among EAs seems to be veg*n (vegetarian/vegan) advocacy.  But there's a big problem with veg*n advocacy: most people who switch to veg*n diets later switch back.  Animal Charity Evaluators writes:

Overall, the evidence suggests that the vast majority of people who adopt dietary changes like vegetarianism, veganism, and other forms of animal-product-limiting eventually return at least some distance towards their former dietary habits. An intervention that changes an individual’s diet usually has only a temporary effect.

Based on the data, ACE estimates that the average new veg*n lasts about 7 years, and they consider this an important factor affecting the cost-effectiveness of interventions to promote veg*nism.  But I'd submit that veg*n recidivism is actually even more important than ACE realizes.

EDIT: I highly recommend this comment to read along with the rest of this post.

Shifting the net growth rate of the veg*n movement

If a quantity is growing exponentially, the rate of growth is much more important in the long run than the quantity's current value.  A veg*n movement that constitutes 1% of the population and reliably doubles in size every 2 years will quickly surpass a veg*n movement that constitutes 10% of the population and reliably doubles in size every 20 years.

Exponential growth seems like a good way to think about the growth of the veg*n movement because the growth rate of the veg*n movement is related in an important way to the movement's current size.  A larger movement means more word of mouth and more resources to plow back in to movement growth, creating a positive feedback loop.  And indeed, Google searches for the term "vegan" seem to be on a rough exponential trend, having doubled in the past 4-5 years.

Exponential growth gets harder and harder to sustain the more you grow, and I think it'd be wishful thinking to say that the current exponential trend will continue forever.  The animal rights movement has been around for decades and the vast majority of the population is still omnivorous.  But the exponential model is still a powerful way to think about veg*n movement growth.

Based on the exponential model, it makes sense to pay less attention to the current size of the veg*n movement and more attention to the net rate of growth in the veg*n movement.  And there are two factors affecting that rate of growth: the rate at which people come in, and the rate at which people leave.

The rate at which people come in roughly corresponds to the quality of veg*n messaging.  Throwing your resources behind a message that converts at a higher rate means a higher rate of growth regardless of how many resources you have.  Fortunately for veg*ns, organizations like The Humane League systematically test different messaging strategies to see which work best.  (But note that more aggressive veg*n marketing strategies may backfire in the long run if they turn some people off of veg*nism and make it so exponential growth tops off sooner.)

On the other hand, many fewer people seem to be working on the other factor that effects the net growth rate of the veg*n movement: the rate at which people leave.  And there are independent reasons to believe this number is more important.  Sam Altman, president of Y Combinator, writes on Twitter:

a much better metric than growth rate for early-stage startup success is how much users love the product.  but it's much harder to measure.

they eventually converge, but it often takes years.

in the mean time, lazy investors use growth rate as a proxy for product love, but often get tricked by growth hackers.

the best investors dig in and figure out the answer to the first-order question.

To someone like Sam, the current veg*n movement might look like a startup that has a bunch of talented growth hackers but an uninspired product.  Most users abandon it.  In fact, it's even true that the majority of self-described "vegetarians" eat meat.

Why is veg*n recidivism so high, and what can be done to decrease it?

Why do most vegetarians go back to eating meat? According to data collected by Hal Herzog the major reasons are, in order of frequency, declining health, hassles and social stigma, irresistible urges and shifts in moral thinking. Interestingly only 2/77 reported shifts in moral thinking made them go back to eating meat. This jives with what I’ve read about former vegans. From the small sample of ex-vegans interviews I’ve read, most seem to experience cravings or health problems and then rationalize their choices with changes in moral ideology.

From here.  Animal Charity Evaluators writes:

It was also found that half of all veg*ns replaced meat with more vegetables, where legumes and grains would be a better choice nutritionally.

So it seems that the veg*n movement is not doing a very good job of helping veg*ns get all the nutrients they need in the absence of animal products.

To delve in to this question further, it might be interesting to try to understand whether veg*n recidivism is higher in areas that have more veg*n food options (likely confounded with the number of veg*ns living in an area and providing social support).

How might the veg*n movement work to reduce recidivism?  These blog posts on the ethics of eating oysters and mussels seems like a good step.  Educating veg*ns on how to get nutrients like creatine and choline that are mostly found in animal products could be valuable.  There might also be value in copying elements of traditional, and therefore proven, vegetarian and pescetarian diets.

Why has the veg*n movement neglected recidivism thus far?  Note that many veg*ns insist that maintaining a veg*n diet doesn't require much ongoing effort.  I think the evidence shows pretty definitively that there's a large segment of the population that finds it difficult.  So perhaps veg*ns are self-selected to find veg*nism easy.

Veg*n recidivists leave the veg*n movement, leading to them being underrepresented in the movement's decisionmaking.  The leadership of the veg*n movement is thus made up entirely of non-recidivists.

Conclusion

As you might have guessed, veg*nism advocacy isn't my current guess for the best cause and I don't restrict my diet.  (I won't get in to details on this because I'd prefer to keep the discussion focused.)  But I know I appreciate people providing informed, thoughtful outside criticism of the causes I support, and I guessed many veg*n Effective Altruists feel the same way, so that's what I tried to do for them.  Even though I don't think veg*n advocacy is the best cause, being veg*n seems admirable from my perspective and I thank veg*ns for their commitment to sentient life.