These numbers are encouraging and exciting, but the immediate question is: Why? Why are some attitudes changing and not others? And also, who is changing? Is everyone in America changing at the same rate, regardless of age or gender or sexual orientation? Or are there some specific groups — perhaps just liberals — that are driving this change? We started to answer those questions in the current paper, and are diving into them in a second paper we hope to have soon.

So that's what we found for implicit attitudes, and that was really exciting because it was one of the first suggestions that change might be happening. We also found for explicit attitudes that all six — attitudes toward sexuality, race, skin tone, age, disability and body weight — have changed towards neutrality over the past decade.

Contrary to the assumption that implicit attitudes don't change, we found that actually, three out of the six implicit attitudes have shown change. Sexuality attitudes, at an implicit level, have changed by about 33 percent over the past decade, whereas race and skin-tone perceptions also changed, just at a slower rate: about 17 percent for race attitudes and 15 percent for skin tone attitude.

Charlesworth says the study tapped into data collected over the last 20 years on the Project Implicit site, which lets visitors test themselves for biases. It looked for changes in attitudes toward six social groups: on the basis of race, skin tone, sexual orientation, age, disability and body weight. Here's our conversation, edited for clarity:

And it showed that implicit biases, initially thought to be so deep-seated that they were immutable, can change over time, and toward multiple groups of people.

The study also found a big drop in racial bias — but a rise in bias against people based on body weight.

It finds that attitudes about sexual orientation changed the fastest, says lead author Tessa Charlesworth: "The most striking finding is that sexuality attitudes have changed toward neutrality, toward less bias, by as much as 33 percent on implicit measures," and by 49 percent on explicit measures — people's reports about their own attitudes.

The study in the journal Psychological Science looks at more than 4 million online tests for implicit bias — bias people aren't aware of — taken from 2007 to the end of 2016.

New research from Harvard University finds that Americans' unconscious bias on the basis of sexual orientation and race dropped dramatically over a decade.

The decline in anti-gay bias seems to be more of a cultural or period effect, something that's happening to everyone in society, rather than just to specific groups.

The surprising thing was that for sexuality attitudes, which have shown the largest change over time, we actually see that it's cross-cutting across all demographic groups — across millennials, across baby boomers, and also across gay, lesbian and straight individuals as well.

So it seems to be more of a cultural or period effect, something that's happening to everyone in society, rather than just to specific groups.

And in terms of race and skin tone?

Those changes tend to be more specific. They seem to be happening most strongly for millennials, so the younger generation seems to be driving the change. There was slower change among Generation Xers and baby boomers.

And white Americans were changing, but black Americans actually haven't changed that much over time. Black Americans haven't moved towards greater pro-black preference at the same rate that white Americans have moved towards pro-black preference. That's one thing that I think should be explored further: Why is it that some groups are changing more than others?

And what about body weight?

Explicitly, it's changing the slowest of all explicit attitudes. That was surprising in and of itself. Implicitly, it showed an even starker difference: It is the only attitude out of the six that we looked at that showed any hint of getting more biased over time.

That was particularly true in the early years that we were looking at: from about 2004 to about 2010, body weight bias actually increased by 40 percent. So almost as large of a change in the opposite direction from sexuality attitudes.

And that to us is striking because it counters the simple narrative that everything's getting better. There are some things that are getting worse. And of course again the question might be: why? What is specific about body-weight attitude?

We can only speculate: Body weight has been the target of much discussion, but discussion in a negative light. We often talk about the 'obesity epidemic,' or about 'the problem' with obese individuals.

Also, we typically think about body weight as something that people can control, and so we are more likely to make the moral judgement of, 'Well, you should just change.'

These numbers describe the change over time in implicit biases, but what are the absolute numbers?

Essentially, we find that the percentage of respondents showing weak, moderate, or strong pro-straight (vs. gay or lesbian) preferences declined from 67 percent of respondents in 2007 to 56 percent in 2016.

Pro-white (vs. black) preferences declined from 67 percent of respondents to 63 percent, and pro-light skin (vs. dark skin) preferences declined from 69 percent of respondents in 2007 to 64 percent of respondents in 2016.

However, the percentage showing pro-thin (vs. fat) preferences increased from 75 percent to 81 percent in 2016.

In contrast, the percentage showing any pro-young (vs. old) preferences remained much more stable at 77-78 percent, and the pro-abled (vs. disabled) remained around 78-80 percent.

Do you expect any change — say, some sort of 'Trump effect,' in these trend lines?

I'm hesitant to make any strong claims. Past analyses with this data looked at whether there was significant change that happened through Obama's presidency. And they found that there was actually no meaningful flip or "breakpoint" that happened as a consequence of Obama's election. We would've expected really significant consequences — the first African-American president, a big change in the national conversation — and that doesn't seem to have been the case.

So what might best explain these findings on the drops in bias?

Again, it's really speculation, but on the sexuality attitudes, for instance, this really is a moment for gay and lesbian rights in America. The change in sexuality attitudes really seems to be an outlier in terms of how rapidly they've changed over time.

We think that a lot of that could have to do with national changes in legislation and social policies. Marriage equality has been a huge push in recent years. And these conversations that are pervading the national rhetoric about sexuality attitudes are very positively valenced. We're almost always talking about rights being granted to gay and lesbian individuals, marriage equality being granted. And I think that the the degree of conversation that's happening, coupled with the fact that it's often positive conversation, has been really powerful in shifting the attitudes.

What about race?

Race has also changed, a little bit slower. And it's also a hugely contentious topic of debate. But in contrast to what we saw with sexuality, a lot of the conversations that we've had in the nation over the past few years have been somewhat more negative in valence.

So if you think about some of the most powerful movements that have happened for African-American rights, they've been things like the Black Lives Matter movement. And they've been talking about rights being taken away, about people dying or being killed by police, the use of lethal force.

Those are conversations that we need to have, because these are really horrific things, but they're very different from the types of conversations that we have been having about sexuality. And that difference in terms of what we're focusing on in our conversation might have a consequence for how our attitudes are changing.

More broadly, what do these findings — which there's good statistical reason to think accurately reflect attitudes in society at large — tell us?

In the past, we thought that implicit attitudes were some stable feature of society or some stable feature of an individual's mind. But this is the first demonstration across multiple attitudes that implicit attitudes have the capacity for long-term durable shifts.

Showing that change is possible opens up a whole new arena for research, in understanding why, how, for whom, is this always the case, and what is the mechanism. So it's really exciting to show potential for attitudes to change, because now we can try and understand why.