Last spring, Daniel Stokols, PhD, of the University of California–Irvine, video recorded his environmental psychology lectures as a first foray into developing an online course. A proponent of face-to-face interactions with students, Stokols saw the videos as a useful supplement to classroom-based learning, not a substitute for it. He uploaded the videos to iTunes U, a component of the iTunes music store that features free academic content, and proceeded with his course as usual. "I thought maybe 100 people would view the course," he says. Fast forward a few months. Apple had featured Stokols's course on its iTunes U homepage. By late July, the course had more student enrollments per week than any other. By September, it had reached 100,000 subscribers and, for months, it remained one of iTunes U's top 10 courses. In November, subscriptions topped 170,000 students.

"It's mind-boggling," says Stokols, who has taught at UCI for nearly 40 years and has "never come close" to reaching that many students. Now, he's heard from a photographer in Germany who says the course has changed the way she interprets her photos; a nurse anesthesia student in Pittsburgh who learned that surgical patients require less pain medication if their beds face windows; and a professor in China who had never heard of environmental psychology before.

"The gratifying part is the feedback from people around the world who are enjoying the material and finding it useful," he says.

Stokols is just one of a growing number of professors turning to iTunes U to host content for their students and share high-quality educational material with the public. Apple launched the platform in 2007; Stanford, UC-Berkeley, MIT and Duke were among the first to sign on. Professors can upload syllabi, handouts, quizzes, slides and links to online resources in addition to audio and video lectures. Students, professionals and curious laypeople can access the courses for free via their computers (PCs included) or with an iPod, iPhone or iPad. The iOS application, which launched in January 2012, had been downloaded more than 14 million times by the end of the year.

"What's really heartening is … how hungry people are for good science," says Dacher Keltner, PhD, of UC Berkeley, whose "Psychology of Emotion" course was listed as one of the top five educational downloads in Wired magazine. He regularly gets feedback from listeners, including an employee at a cardboard box-making factory who's found that the lectures make his job bearable and a retired attorney who said that if he had known about the science of emotion earlier, his professional life and marriage would have been different.

Many universities don't appear to have a problem with providing the public with the same content –– but not the credits –– that students pay big bucks for. At Yale, for example, Fred Volkmar's course on autism serves as a reliable source in an Internet pool diluted with millions of websites selling cures for autism. "It's very consistent with Yale's vision of wanting to get quality product out there," he says.

Below is a sampling of some of the psychology courses available on iTunes U. To access the courses, download the iTunes app or go to the iTunes store.

"Environmental psychology"

University of California, Irvine

Instructor: Daniel Stokols, PhD, chancellor's professor of social ecology in the departments of psychology and social behavior, and planning, policy and design

Why tune in? Environmental psychology is about how we're influenced by our everyday surroundings, including our offices, dorm rooms, commutes and exposures to nature. Stokols's course addresses a variety of issues, including how the design of an apartment influences the formation of friendships, why people litter and the consequences of a society suffering from information overload. "Today, there's so much concern about issues of sustainability, public health, pollution and population growth that viewing the world as a system and in ecological terms … is very timely," says Stokols.

Fun fact from the course: People are more likely to throw away trash in a garbage can that's painted decoratively than one that's plain.

"Health psychology"

American University

Instructor: Brian Yates, PhD, professor of psychology

Why tune in? Yates originally intended iTunes U to serve as a resource for his own students, who are challenged to evaluate and change their own habits to promote health. They assess their personal risks, identify what they want to change and maintain, and set up a system of "triggers and flags" that will signal when it's time to seek professional help in the future. The material has caught on — his course is consistently one of the top 10 downloaded from iTunes U and had more than 35,000 enrollees in October. "The field is very exciting. It's young, dynamic, it affects every one of us," says Yates. "That's what psychology is supposed to do."

Surprising fact from the course: One study of HIV-positive men found that those who tended to blame themselves for negative outside events experienced a significantly faster decline in helper T cells, important for maintaining immune function.

"Human emotion"

University of California, Berkeley

Instructor: Dacher Keltner, PhD, professor of psychology and director of Berkeley's Social Interaction Laboratory

Why tune in? Keltner's course has always been well-attended, so it was Berkeley's idea to make it available to the public through iTunes U. The course details fascinating research on art and emotional expression, cultural similarities and differences in non-verbal expressions, and emotion's neurobiological and hormonal underpinnings. "The study of human emotion is new, it's growing and it's relevant to people around the world," says Keltner.

Interesting fact from the course: People can usually accurately convey — and interpret — emotion through nothing more than a brief touch. But in a study conducted by Keltner and his team, there were two instances in which the "touchee" was clueless: When women tried to convey anger to men, and when men tried to communicate sympathy to women. "That fits how emotions are gendered, and how families socialize women into the ways of sympathy and men into the ways of anger that might account for these differences," he says.

"Autism and related disorders"

Yale University

Instructor: Fred Volkmar, MD, chief of child psychiatry at Yale-New Haven Children's Hospital

Why tune in? A rotating panel of mental health experts lead this course on the latest autism research, including a lecture by Volkmar's co-instructor, James McPartland, PhD, that details how brain electrophysiology is informing researchers' understanding of social perception in autism. "This is a happy story in the sense that outcomes seem to be getting better with early intervention and protection," says Volkmar, who estimates the course's first lecture has gotten about 21,000 views on iTunes.

Interesting fact from the course: One of the early theories of autism speculated that intelligent parents were more likely to have autistic children. But the idea was likely a selection bias: The people who knew about what's now known as autism were predominantly researchers or other academics. "Now," he says, "you see children with autism everywhere … from all social classes, from every continent on the globe — and it looks remarkably the same. What's different is how people respond to it."

"Great ideas in psychology"

Missouri State University

Instructor: Todd Daniel, professor of psychology and director of Missouri State's RStats (Research, Statistical Training, Analysis and Technical Support) Institute

Why tune in? Daniel is a former radio producer who uses his storytelling skills to bring psychology to life in this introductory course. The course, which is Missouri State's most downloaded podcast, begins with the "Myth of Psyche" and takes the listener through an engaging overview of psychology including lectures on dreaming and hypnosis, a health course dubbed "Why College Is Bad for You" and the truth about Freud. "After I do a lecture in front of a seated class, when it's over, it's gone like a vapor," says Daniel. "I wanted to create something more permanent."

Surprising fact from the course: In 1964, a man named Randy Gardner went 264 hours, or about 11 days, without sleep. He was trying to prove that sleep wasn't all that important, but the changes noted in his cognitive and behavioral functioning proved otherwise — a lesson Daniel tries to impart on his students. "Your best strategy is to get a good night's sleep," he says.