The question is simple - what are the biggest ways that people in society are acting more rationally since the start of the third millenium?
1. A/B Testing - Testing two products and seeing which one people like better has been around since New Coke improperly replaced Coca Cola Classic. What is different in the third millenium is that with so much business being done online it's both easier and cheaper to test various options against each other. Different pages for customer check out, different email marketing campaigns and many other approaches can be tested against each other randomly, quickly and cheaply. Zynga is well known for doing heavy A/B testing in its monetization, retention and recruitment of online casual gamers (unfortunately for them what worked best was some variation of massively spamming online friends or their own Facebook wall in order to receive power ups in game, which Facebook cut down on). At the very least, randomized controlled trials are gaining a much stronger foot in the business community than every before.
2. Rationality communities and rationality materials have spread a lot thanks to the internet. We have websites like overcomingbias.com, lesswrong.com and lists of common biases at the tip of our fingertips. Before the access to the type of material on these sites was restricted to academia, libraries and some relatively specific social groups. Finding books on rationality has also never been easier.
3. Smart phones have rendered debates about most facts relatively useless. If people are wondering who was actually president was in 1969 it's really easy to figure out in a few seconds. Before ubiquitous access to the internet (Google and Wikipedia), people who disagreed about common factual matters would have to share related facts ("Don't you remember that Nixon called Buzz Aldrin and Neil Armstrong while they were on the moon? The moon landing was also in 1969.") and hope that they could come to a common world view through agreement on peripheral facts. That process has been largely replaced by the internet giving everyone access to most common facts.
4. Scholarly research is more available than ever before. Anyone searching for articles will find that there are still many pay walls, but there is progress being made to open up research access to the general public. However, until old papers like JSTOR also open up the references to older papers in newly published work will still leave those without access behind the pay-wall in the relative dark.
5. Prediction markets. People have started trusting prediction markets more than most experts, though there have been a few exceptions. Idea futures and expansions on them such as state contingent idea futures has been a pet idea of Robin Hanson for some time. Regulators haven't been too friendly and/or the market place isn't quite liquid enough for most markets, so while it is interesting that there are places out there which curates an active market for people who want to put their money where their mouth is, a lot more would have to happen on this front for it to be truly significant.
6. Much better analytical tools. I may be a bit biased for even including this idea on the list, but with AI largely disappointing (though many people are now finding a tool like Siri useful), the ability for people to utilize computers in their analysis has vastly increased in the past decade. With the evolution of tools such as Palantir, it is becoming much easier to look through the connections between large amounts of data.
7. Better ways to organize and present information. Wikipedia, reddit-type threaded message boards and task organizers like Asana and programming version control such as git have been quite useful. Blogs by experts are also very interesting, and their ability to crowd-source their readership for ideas/answers/corrections should not be underweighted. The investors in Quora certainly think this space is valuable. There are enough ideas in here that it could have been at least a few points on the list.
There have also been ways in which rationality has decreased - more and more of public, private and scientific life is becoming politicized. But reviewing the above list makes me a bit optimistic. While not the most important, I think that the rise of A/B testing is probably the most underrated trend, while #3 and #7 should have the widest impact.
Hat tip: Kevin Simler for some of the ideas mentioned above