How many times does the chorus repeat in your favorite song?
And, take a moment to think, how many times have you listened to it?
Chances are you've heard that chorus repeated dozens, if not hundreds, of times, and it's not just popular songs in the West that repeat a lot.
Repetition is a feature that music from cultures around the world tends to share.
So, why does music rely so heavily on repetition?
One part of the answer come from what psychologists call the mere-exposure effect.
In short, people tend to prefer things they've been exposed to before.
For example, a song comes on the radio that we don't particularly like, but then we hear the song at the grocery store, at the movie theater and again on the street corner.
Soon, we are tapping to the beat, singing the words, even downloading the track.
This mere-exposure effect doesn't just work for songs.
It also works for everything from shapes to Super Bowl ads.
So, what makes repetition so uniquely prevalent in music?
To investigate, psychologists asked people to listen to musical compositions that avoided exact repetition.
They heard excerpts from these pieces in either their original form, or in a version that had been digitally altered to include repetition.
Although the original versions had been composed by some of the most respected 20th century composers, and the repetitive versions had been assembled by brute force audio editing, people rated the repetitive versions as more enjoyable, more interesting and more likely to have been composed by a human artist.
Musical repetition is deeply compelling.
Think about the Muppets classic, "Mahna Mahna."
If you've heard it before, it's almost impossible after I sing, "Mahna mahna," not to respond, "Do doo do do do."
Repetition connects each bit of music irresistibly to the next bit of music that follows it.
So when you hear a few notes, you're already imagining what's coming next.
Your mind is unconsciously singing along, and without noticing, you might start humming out loud.
Recent studies have shown that when people hear a segment of music repeated, they are more likely to move or tap along to it.
Repetition invites us into music as imagined participants, rather than as passive listeners.
Research has also shown that listeners shift their attention across musical repetitions, focusing on different aspects of the sound on each new listen.
You might notice the melody of a phrase the first time, but when it's repeated, your attention shifts to how the guitarist bends a pitch.
This also occurs in language, with something called semantic satiation.
Repeating a word like atlas ad nauseam can make you stop thinking about what the word means, and instead focus on the sounds: the odd way the "L" follows the "T."
In this way, repetition can open up new worlds of sound not accessible on first hearing.
The "L" following the "T" might not be aesthetically relevant to "atlas," but the guitarist pitch bending might be of critical expressive importance.
The speech to song illusion captures how simply repeating a sentence a number of times shifts listeners attention to the pitch and temporal aspects of the sound, so that the repeated spoken language actually begins to sound like it is being sung.
A similar effect happens with random sequences of sound.
People will rate random sequences they've heard on repeated loop as more musical than a random sequence they've only heard once.
Repetition gives rise to a kind of orientation to sound that we think of as distinctively musical, where we're listening along with the sound, engaging imaginatively with the note about to happen.
This mode of listening ties in with our susceptibility to musical ear worms, where segments of music burrow into our head, and play again and again, as if stuck on repeat.
Critics are often embarrassed by music's repetitiveness, finding it childish or regressive, but repetition, far from an embarrassment, is actually a key feature that gives rise to the kind of experience we think about as musical.
Wars are a tragic part of our history and will almost certainly be a tragic part of our future.
Since the establishment of the United Nations, wars of aggression have been outlawed and multilateral conventions refer to armed conflict instead of war.
But the wars of the future won't be like the wars of our past.
Alongside traditional warfare, our future will include cyberwarfare, remotely fighting our enemies through the use of a new class of weapons, including computer viruses and programs to alter the enemy's ability to operate.
And not only is cyberwarfare not covered by existing legal frameworks, but the question of what exactly constitutes cyberwarfare is still highly debated.
So, how can we deal with cyberwarfare if we can't even agree on what it means?
One way forward is to envision situations where new international laws may be needed.
Imagine a new kind of assassin, one that could perpetrate a crime without firing a single shot or even being in the same country.
For example, an individual working for the government uses a wireless device to send a signal to another foreign leader's pacemaker.
This device directs the pacemaker to malfunction, ultimately resulting in the foreign leader's death.
Would this cyber assassination constitute an act of war?
As a second example, imagine an allied group of nations cooperatively infiltrating the computer systems of an enemy nation's nuclear warship.
This attack results in a nuclear-powered aircraft carrier almost melting down, which was stopped just short of killing thousands of soldiers and civilians.
As a defensive measure, the enemy country responds by unleashing a defensive cyberattack that results in the allied nations' power grids going down.
Hospitals can no longer treat patients, entire regions without heat or clean water, all ultimately causing tens of thousands civilian deaths.
The origin of the power failure was the counterattack, but the fragile infrastructure, feeble cybersecurity, and the antiquated state of the power grid all contributed to the deaths of the civilians.
Could the country fight back?
Who would they fight?
And would their retaliation be considered an act of war?
Do they constitute war crimes against humanity?
Who is to be held responsible?
The computer programmers who wrote the code?
The military project manager who oversaw the creation of the code?
The commander who hit the button, setting off the event?
The hardware engineer who created the computers, knowing that they were intended to enable an attack?
Because war has been with us for so long, we have laws to deal with figuring out who should be held accountable for their actions in combat.
These legal frameworks aim to contain and prevent atrocities from being more atrocious.
Commandeering civilian planes and using them as weapons, dropping atomic bombs, the use of gas chambers or poisonous gas in conflict, all of these actions, if committed, constitute acts of war and war crimes under customary international law and the Hague conventions.
Again, the current legal framework stays silent on hypothetical questions and countless others because there are no easy answers, and there are only two ways to make progress on these questions: peace or new laws.
So, what hypothetical but plausible scenarios can you imagine falling under the burgeoning definition of cyberwarfare, and how might you design an international legal framework to deter these activities?
I am a reformed marketer, and I now work in international development.
In October, I spent some time in the Democratic Republic of Congo, which is the [second] largest country in Africa.
In fact, it's as large as Western Europe, but it only has 300 miles of paved roads.
The DRC is a dangerous place.
In the past 10 years, five million people have died due to a war in the east.
But war isn't the only reason that life is difficult in the DRC.
There are many health issues as well.
In fact, the HIV prevalence rate is 1.3 percent among adults.
This might not sound like a large number, but in a country with 76 million people, it means there are 930,000 that are infected.
And due to the poor infrastructure, only 25 percent of those are receiving the life-saving drugs that they need.
Which is why, in part, donor agencies provide condoms at low or no cost.
And so while I was in the DRC, I spent a lot of time talking to people about condoms, including Damien.
Damien runs a hotel outside of Kinshasa.
It's a hotel that's only open until midnight, so it's not a place that you stay.
But it is a place where sex workers and their clients come.
Now Damien knows all about condoms, but he doesn't sell them.
He said there's just not in demand.
It's not surprising, because only three percent of people in the DRC use condoms.
Joseph and Christine, who run a pharmacy where they sell a number of these condoms, said despite the fact that donor agencies provide them at low or no cost, and they have marketing campaigns that go along with them, their customers don't buy the branded versions.
They like the generics.
And as a marketer, I found that curious.
And so I started to look at what the marketing looked like.
And it turns out that there are three main messages used by the donor agencies for these condoms: fear, financing and fidelity.
They name the condoms things like Vive, "to live" or Trust.
They package it with the red ribbon that reminds us of HIV, put it in boxes that remind you who paid for them, show pictures of your wife or husband and tell you to protect them or to act prudently.
Now these are not the kinds of things that someone is thinking about just before they go get a condom.
(Laughter) What is it that you think about just before you get a condom?
Sex!
And the private companies that sell condoms in these places, they understand this.
Their marketing is slightly different.
The name might not be much different, but the imagery sure is.
Some brands are aspirational, and certainly the packaging is incredibly provocative.
And this made me think that perhaps the donor agencies had just missed out on a key aspect of marketing: understanding who's the audience.
And for donor agencies, unfortunately, the audience tends to be people that aren't even in the country they're working [in].
It's people back home, people that support their work, people like these.
But if what we're really trying to do is stop the spread of HIV, we need to think about the customer, the people whose behavior needs to change -- the couples, the young women, the young men -- whose lives depend on it.
And so the lesson is this: it doesn't really matter what you're selling; you just have to think about who is your customer, and what are the messages that are going to get them to change their behavior.
It might just save their lives.
Thank you.
(Applause)