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How Do Neural Implants Work?

Par Emily Waltz

It sounds like science fiction, but a neural implant could, many years from now, read and edit a person’s thoughts. Neural implants are already being used to treat disease, rehabilitate the body after injury, improve memory, communicate with prosthetic limbs, and more. 

The U.S. Department of Defense and the U.S. National Institutes of Health (NIH) have devoted hundreds of millions of dollars in funding toward this sector. Independent research papers on the topic appear in top journals almost weekly.

Here, we describe types of neural implants, explain how neural implants work, and provide examples demonstrating what these devices can do. 

What is a neural implant? 

A neural implant is a device placed inside the body that interacts with neurons. 

Neurons are cells that communicate in the language of electricity. They fire electrical impulses in particular patterns, kind of like Morse code. An implant is a human-made device that is placed inside the body via surgery or an injection.

Illustration conceptualizing a neuron firing
Gif: Spencer Sutton/Science Source
When a neuron is activated, it fires an electrical impulse that can be recorded by an electrode.

A neural implant, then, is a device—typically an electrode of some kind—that’s inserted into the body, comes into contact with tissues that contain neurons, and interacts with those neurons in some way. 

With these devices, it’s possible to record native neural activity, allowing researchers to observe the patterns by which healthy neural circuits communicate. Neural implants can also send pulses of electricity to neurons, overriding native firing patterns and forcing the neurons to communicate in a different way. 

In other words, neural implants enable scientists to hack into the nervous system. Call it neuromodulation, electroceuticals, or bioelectronics—interventions involving neural implants have the potential to become tremendously powerful medical tools. 

Consider the functions of the nervous system: It controls thinking, seeing, hearing, feeling, moving, and urinating, to name a few. It also controls many involuntary processes such as organ function and the body’s inflammatory, respiratory, cardiovascular, and immune systems.

“Anything that the nervous system does could be helped or healed by an electrically active intervention—if we knew how to do it,” says Gene Civillico, a neuroscientist at the NIH, who runs the agency’s peripheral nerve stimulation funding program SPARC

How are neural implants used? 

One of the most established clinical uses of neural implants is in a treatment called deep brain stimulation, or DBS. In this therapy, electrodes are surgically placed deep into the brain where they electrically stimulate specific structures in an effort reduce the symptoms of various brain-based disorders.

Medtronic diagram illustration of DBS therapy for epilepsy
Illustration: Reprinted with the permission of Medtronic
Medtronic’s DBS system is most commonly used today to treat the movement symptoms of Parkinson’s disease.

The U.S. Food and Drug Administration (FDA) first approved the use of DBS in 1997 for essential tremor. Since then, the FDA or other global regulators have approved DBS for Parkinson’s disease, dystonia, tinnitus, epilepsy, obsessive-compulsive disorder, and neuropathic pain. DBS is also being investigated as a treatment for Tourette syndrome and psychiatric disorders such as depression. It is estimated that more than 150,000 people globally have received a DBS implant. 

Researchers have also put a great deal of time into manipulating the vagus nerve using neural implants. The vagus nerve connects most of our key organs to the brain stem, and researchers are hacking this communication superhighway in an effort to treat heart failure, stroke, rheumatoid arthritis, Crohn’s disease, epilepsy, type 2 diabetes, obesity, depression, migraine, and other ailments.   

In 2018, patients who had been paralyzed due to spinal cord injuries walked again with the help of spinal stimulation.  

Some of the most emotionally moving experiments involving neural implants have come with the stimulation of the spinal cord, also known as epidural stimulation. The treatment has enabled a handful of people with paralysis in their lower bodies to move, stand, and even walk a short distance for the first time since sustaining spinal cord injuries. 

Perhaps no neuromodulation research has captivated the public’s imagination more than mind-controlled prostheses. These systems enable amputees to control robotic hands, arms, and legs—in rudimentary ways—using their thoughts. This can be accomplished with a neural implant in the brain or in the extremity above the amputation. Some of these robotic limbs can also provide sensory feedback by stimulating nerves just above the amputation, giving the user a sense of what he or she is touching

The BrainGate research team has used neural implants to enable people with quadriplegia to control computer cursors and robot arms. 

And then there’s the stuff that comes across like science fiction. Researchers have successfully enhanced people’s memory capability for specific tasks by stimulating brain structures in precise ways. Quadriplegic individuals with brain implants have operated computers and typed sentences using only their thoughts. There’s an algorithm that can determine a person’s mood based on brain activity alone. A couple of companies have successfully brought to market implants that correct neural communication between the eye and the brain. Elon Musk says his company Neuralink plans to sync our brains with AI.

What’s next for neural implants?

The invasiveness of any implant limits its use. It’s hard to justify brain or spinal surgery unless a person is in severe medical need. So engineers are constantly inventing better devices that reach deep in the body with less impact on tissues. 

“Engineers are continually pushing the boundaries for what’s technically possible,” says David McMullen, program chief of the neuromodulation and neurostimulation program at the U.S. National Institute of Mental Health. “It’s all about decreasing the surgical burden, increasing the chronic nature of the implant and constantly trying to get ever smaller electrodes that cover a wider area of brain,” he says. 

Engineers have concocted dust-sized brain implants, electrodes that climb nerves like a vine, electrodes made from flexible materials such as a nanoelectronic thread, stent-like electrodes, or “stentrodes,” that can get to the brain via blood vessels and record electrical activity, injectable electronic mesh made from silicon nanowires, electrodes that can be injected into the body as a liquid and then harden into a stretchy taffy-like substance, and more.  

“Stentrodes” could be inserted into the brain via blood vessels, obviating the need for open-brain surgery. 

Neuromodulation can even be performed non-invasively using electrodes or magnetic coils placed on or near the skin. The strategy has proven effective for some conditions, although so far it doesn’t have the specificity or efficacy of implants.

But these innovative devices only get us so far. “There’s a misconception that the obstacles [to neuromodulation] are mainly technical, like the only reason we don’t have thought-controlled devices is because nobody has made a flexible-enough electrode yet,” says Civillico at NIH.

Researchers still need a basic understanding of the physiology of neural circuits, says Civillico. They need maps of how neurons are communicating, and the specific effects of these circuits on the body and brain. Without these maps, even the most innovative implants are effectively shooting electrical impulses into the dark. 

AI-Designed 'Living Robots' Crawl, Heal Themselves

Par Megan Scudellari

Biological organisms have certain useful attributes that synthetic robots do not, such as the abilities to heal, adapt to new situations, and reproduce. Yet molding biological tissues into robots or tools has been exceptionally difficult to do: Experimental techniques, such as altering a genome to make a microbe perform a specific task, are hard to control and not scalable.

Now, a team of scientists at the University of Vermont and Tufts University in Massachusetts has used a supercomputer to design novel lifeforms with specific functions, then built those organisms out of frog cells.

The new, AI-designed biological bots crawl around a petri dish and heal themselves. Surprisingly, the biobots also spontaneously self-organize and clear their dish of small trash pellets.

“This wasn’t something that we explicitly selected for in our evolutionary algorithm,” says Josh Bongard, a roboticist at the University of Vermont who co-led the research, published this week in the Proceedings of the National Academy of Sciences. “It emerges from the fact that cells have their own intelligence and their own plans.”

The idea for AI-designed biobots came from a DARPA funding call for autonomous machines that adapt and thrive in the environment. Bongard and biologist Michael Levin at Tufts University conceived a plan to take advantage of Mother Nature’s hard work and build a machine out of something already capable of adapting: living cells.

The researchers ran an evolutionary algorithm on a supercomputer at the University of Vermont over several days. The algorithm, inspired by natural selection, used biological building blocks to create a random population of new life-form candidates. The algorithm then winnowed through the designs with a fitness function that scored each candidate on its ability to do a certain thing—in this case, the ability to move.

The most promising designs became the basis to spawn a new set of designs, and the best of those were selected again. Rinse and repeat, and after 100 runs of the algorithm, tossing out billions of potential designs, the team had a set of five finalists—AI-created designs that moved well in silico.

Bongard’s team sent the finalist designs to Levin’s lab at Tufts, where microsurgeon Douglas Blackiston deemed four of the five designs too difficult or impossible to build. But the fifth design seemed doable. Blackiston used tiny forceps and a tiny electrode under a microscope to cut and join heart and skin cells from the African frog Xenopus laevis into a close approximation of the computer’s design. When cut in half, the cells stitched themselves back together—something today’s robots and computers clearly don’t do.

Once constructed, the millimeter-wide biobots moved around a petri dish as the heart cells contracted. When the team put small pellets into the dish, the cells unexpectedly worked together to clump the pellets into neat piles.

Bongard imagines a future where such biobots could be used to clean up microplastics in the ocean, especially as the biobots are 100 percent biocompatible and degrade in salt water. “That might make these biobots a uniquely appealing approach for environmental remediation,” says Bongard.

For now, the miniscule robots are best at locomotion, but Bongard has other tasks in mind. The next step, he says, is developing a “cage bot”—an empty cube to pick up and carry a payload. With that ability, one could build bots out of a person’s own cells, then use them to deliver medications deep into the body without prompting an immune response, the authors suggest.

Without a digestive system to ingest food or a nervous system to sense the surrounding environment, the organisms lived for just days. In the future, incorporating different cell types could change that: “If we wanted them to exist for longer periods of time, we might want them to be able to find and eat food sources,” says Bongard. “We’d also like to be able to incorporate sense organs into these biobots.” The collaborators are now building AI-designed biobots with mammalian cells.

The team is keenly aware that their new organisms might leave some people feeling unsettled, slipping into the uncanny valley. “Frogs that are not frogs definitely qualify for this,” says Bongard.

Plus, as they create new lifeforms—with, say, digestive, nervous, and even reproductive systems—the team is working with bioethicists and following strict animal welfare laws. “As we move further and further away from recognizable organisms, we may need to create new regulations for this kind of technology,” says Bongard.

Are Your Students Bored? This AI Could Tell You

Par Emily Waltz
Journal Watch report logo, link to report landing page

A professor finishes a lecture and checks his computer. A software program shows that most students lost interest about 30 minutes into the lecture—around the time he went on a tangent. The professor makes a note to stop going on tangents.

The technology for this fictional classroom scene doesn’t yet exist, but scientists are working toward making it a reality. In a paper published this month in IEEE Transactions on Visualization and Computer Graphics, researchers described an artificial intelligence (AI) system that analyzes students’ emotions based on video recordings of the students’ facial expressions.

The system “provides teachers with a quick and convenient measure of the students’ engagement level in a class,” says Huamin Qu, a computer scientist at the Hong Kong University of Science and Technology, who co-authored the paper. “Knowing whether the lectures are too hard and when students get bored can help improve teaching.”

Qu and his colleagues tested their AI system in two classrooms consisting of toddlers in Japan and university students in Hong Kong. The teachers for each class received a readout of the emotions of individual students and the collective emotions of the group as a whole during their lectures.

The visual analytics system did a good job of detecting obvious emotions such as happiness. But the model often incorrectly reported anger or sadness when students were actually just focused on the lectures. (The frown that often washes over our faces when we listen closely can be easily confused, even by humans, with anger, when taken out of context .) “To address this issue, we need to add new emotion categories, relabel our data and retrain the model,” says Qu.

The focus frown and other confusing facial expressions are a challenge for just about everyone working in the field of emotion recognition, says Richard Tong, chief architect at Squirrel AI Learning, who was not involved in the paper. “We have had similar problems in our own experiments,” he says, referring to the multimodal behavioral analysis algorithms his company is developing with its partners.   

Lots of groups are working on some kind of behavior or emotion recognition technology for the classroom, says Tong, who is also the chair of the IEEE Learning Technology Standards Committee. But he says this kind of analysis is of limited use for teachers in traditional classroom settings.

“Teachers are overwhelmed already, especially in the public schools,” Tong says. “It’s very hard for them to read analytical reports on individual students because that’s not what they’re trained for and they don’t have time.”

A group of toddlers have black bars over their eyes and green boxes around their faces.
Photo: Viv Limited

Instead, Tong envisions using emotion recognition and other means of behavioral analysis for the development of AI tutors. These one-on-one computer-based teachers will be trained to recognize what motivates a student and spot when a student is losing interest, based on their physical or behavioral cues. The AI can then adjust its teaching strategy accordingly.

In this world of AI tutors, Tong says he envisions human teachers taking a role as head coach over the AI agents, which would work one-on-one with students. “But that requires a much more capable AI” than what we have now, he says.

Putting video cameras in the classroom also creates privacy issues. “The disclosure of the analysis of an individual’s emotion in a classroom may have unexpected consequences and can cause harm to students,” says Qu.

And it could backfire on educators. “It may distract students and teachers and could be harmful to learning, since students and teachers may feel like someone could be watching them and might not freely express their opinions,” Qu says. “The privacy issue is important for everyone, and needs to be carefully considered.”

10 Tantalizing Tech Milestones to Look for in 2020

Par Michelle Hampson
graphic link to special report landing page
  • illustration leg walking

    Mind-Controlled Bionic Limbs Will Debut in the Boston Marathon

    MIT researchers have developed a way of controlling bionic limbs with thoughts alone. First tried in humans in 2016, the method will be hitting new strides in 2020, when Brandon Korona, a veteran who lost his leg in Afghanistan, uses his new bionic limb to compete in the Boston Marathon. The mind-control technique involves reconstructing muscles near the base of the amputation site and linking them so that the muscles contract and extend in unison. This dynamic interaction and the electrical impulses it generates make it possible for the limb’s processor, which controls the bionic joints, to exchange signals with the brain. This exchange tells the brain where a joint is, how fast it is moving, and what size load it is bearing.

  • illustration of diamond

    Artificial Diamonds Will Really Shine

    Diamonds can cost a lot, in money and even blood, given the sometimes shady ethics of the trade. A solution may lie in lab-grown diamonds. Production of these artificial gems will ramp up considerably in 2020, when one of the world’s largest diamond companies, De Beers Group, opens a manufacturing facility in Oregon that will produce about 500,000 rough carats per year. There, mixed gases and hundreds of chemical substrates will be added to reactors and subjected to high temperatures, transforming carbon into its diamond form. It takes about two weeks to make a 1-carat sparkler, which will retail for about US $800.

  • illustration of storage facility

    Huge Pumped-Storage Project Breaks Ground in Montana

    As the electric grid increasingly relies on wind turbines and solar panels, it requires ever more backup energy to make up for shortfalls on windless or cloudy days. To help address this need, Absaroka Energy will begin construction on a new pumped- storage hydroelectric facility this year, harnessing three massive reservoirs inside the Gordon Butte mountain, in Montana. When electricity is needed, the reservoirs will release water onto three turbine generators below, which together can generate 400 megawatts of electricity. Surplus electricity will be used to pump water back into the reservoirs. The new system uses separate motors to control the pumps, as well as separate turbines and generators. Isolating the components gives the system about 80 percent efficiency.

  • illustration of the Facebook thumbs up icon

    Will a Facebook-Backed Cryptocurrency Overcome Legal Hurdles?

    In July 2019, Facebook and 27 other companies announced plans to release a worldwide cryptocurrency, called Libra. The Libra Association aims to create a safe, stable currency, one that could be especially convenient for the 1.7 billion people globally who don’t have bank accounts. However, the announcement has prompted concerns about how Libra might be used for money laundering and the privatization of money, among other issues. The Libra Association says it is taking proper measures for security and data privacy, an assertion repeated by Facebook CEO Mark Zuckerberg at hearings before the U.S. Congress last October. Meanwhile, seven major companies, including PayPal, Visa, and Mastercard, have left the group. But the association says it still plans to roll out Libra in 2020.

  • illustration of car driving fast

    Tesla’s 2020 Roadster to Hit the Streets

    Here’s some news to get any gearhead’s heart racing: Elon Musk claims that the 2020 Tesla Roadster will be able to rocket from 0 to 97 kilometers per hour (60 miles per hour) in a mere 1.9 seconds. This new machine— not to be confused with Tesla’s 2008–2012 Roadster—will have two motors in the rear and one in the front and offer the option of rocket thrusters powered by compressed air. Aside from the excitement surrounding the Roadster’s acceleration and top speed (which will exceed 200 mph), perhaps the most important spec is range. Currently, the longest range of an electric car is around 600 km (375 miles), but the Roadster will be good for 1,000 km (620 miles).

  • illustration of electric plane

    Rolls-Royce to Fly Record-Breaking Electric Plane

    In the first quarter of 2020, Rolls- Royce will unveil ACCEL, which it says is the fastest all-electric plane ever designed. The company claims its one-seat racing plane should exceed 480 kilometers per hour (300 miles per hour), smashing the current record of 340 km/h, set in 2017 by a Siemens plane. Rolls-Royce and its partners had to monitor more than 20,000 data points per second to optimize the plane’s battery system. An active cooling system allows the battery to discharge at high rates. Look to the skies over Britain to see this plane in action.

  • illustration of hand with nuclear symbol over it

    Lockheed Martin Takes Another Step Toward Compact, Convenient Nuclear Fusion

    Controlled nuclear fusion has been the object of many a failed quest over the past 60 years. Though it could go far to solve the world’s energy needs, the technical demands of fusion power are stupendous. Lockheed Martin has a patent pending on a reactor design that it says has a real chance at success. The reactor is compact, relying on magnetic fields to confine the hydrogen plasma and on electromagnetic fields to ignite and sustain the plasma. This process causes hydrogen atoms to fuse into helium, releasing torrents of energy. Throughout 2020, the company will test the fifth prototype, T5, which it says is significantly more powerful than earlier versions. The tests will show whether the design can handle the immense heat and pressure from the highly energized plasma inside.

  • illustration of auto-mower

    A New Job for a Robot: Mowing

    In 2020, the long wait for a lawn-mowing counterpart to the Roomba will finally be over. iRobot plans to launch Terra, which can mow a lawn in straight, even rows without any human oversight. To navigate, the robot relies on a handful of radio- frequency beacons strategically placed throughout the yard, keeping at least three beacons within its line of sight at all times. By measuring the time it takes for signals to travel between itself and the beacons, Terra can locate itself on a preprogrammed map of the lawn. If anyone tries to steal this hard-working robot, antitheft software registers that the machine has left the premises and renders it inoperable.

  • illustration of turbine

    GE Unveils a Bigger, Better Wind Turbine

    In 2020, GE Renewable Energy will seek certification for its Haliade-X offshore wind turbine, whose rated capacity of 12 megawatts would make it the largest and most powerful on the market. It boasts 107-meter-long blades made of a composite of glass and carbon fiber in a resin matrix. The massive area swept by those blades will let the turbine capture up to 67 gigawatt-hours annually, enough clean energy to power 16,000 households and save up to 42,000 metric tons of CO2. Assuming certification in 2020, sales are expected to commence in 2021.

  • illustration of gaming device

    Google and Apple Compete for the Attention of Gamers

    Gamers will be busy with two gaming services from Apple and Google. Critical to the new services—which launched recently and are expected to sweep the industry in the coming year—is the expansion of bandwidth, in the form of faster Wi-Fi and the emerging 5G capability, both of which greatly reduce lag. For US $4.99 per month, players can access more than 100 games through Apple Arcade, with more being rolled out each month. Google Stadia is available for $9.99 per month, with the option to purchase additional games at up to 4K resolution and at 60 frames per second. While Google Stadia games can be played on a variety of devices, Apple Arcade is, unsurprisingly, available only on Apple devices.

All illustrations: Greg Mably

Caltech’s Brain-Controlled Exoskeleton Will Help Paraplegics Walk

Par Evan Ackerman
graphic link to special report landing page

Photo: Caltech
This lower-body exoskeleton, developed by Wandercraft, will allow disabled users to walk more dynamically.

Bipedal robots have long struggled to walk as humans do—balancing on two legs and moving with that almost-but-not-quite falling forward motion that most of us have mastered by the time we’re a year or two old. It’s taken decades of work, but robots are starting to get comfortable with walking, putting them in a position to help people in need.

Roboticists at the California Institute of Technology have launched an initiative called RoAMS (Robotic Assisted Mobility Science), which uses the latest research in robotic walking to create a new kind of medical exoskeleton. With the ability to move dynamically, using neurocontrol interfaces, these exoskeletons will allow users to balance and walk without the crutches that are necessary with existing medical exoskeletons. This might not seem like much, but consider how often you find yourself standing up and using your hands at the same time.

“The only way we’re going to get exoskeletons into the real world helping people do everyday tasks is through dynamic locomotion,” explains Aaron Ames, a professor of civil and mechanical engineering at Caltech and colead of the RoAMS initiative. “We’re imagining deploying these exoskeletons in the home, where a user might want to do things like make a sandwich and bring it to the couch. And on the clinical side, there are a lot of medical benefits to standing upright and walking.”

The Caltech researchers say their exoskeleton is ready for a major test: They plan to demonstrate dynamic walking through neurocontrol this year.

Getting a bipedal exoskeleton to work so closely with a human is a real challenge. Ames explains that researchers have a deep and detailed understanding of how their robotic creations operate, but biological systems still present many unknowns. “So how do we get a human to successfully interface with these devices?” he asks.

There are other challenges as well. Ashraf S. Gorgey, an associate professor of physical medicine and rehabilitation at Virginia Commonwealth University, in Richmond, who has researched exoskeletons, says factors such as cost, durability, versatility, and even patients’ desire to use the device are just as important as the technology itself. But he adds that as a research system, Caltech’s approach appears promising: “Coming up with an exoskeleton that can provide balance to patients, I think that’s huge.”

The lower-body exoskeleton developed by Wandercraft.img
Photo: Caltech
Caltech researchers prepare for a walking demonstration with the exoskeleton.

One of Ames’s colleagues at Caltech, Joel Burdick, is developing a spinal stimulator that can potentially help bypass spinal injuries, providing an artificial connection between leg muscles and the brain. The RoAMS initiative will attempt to use this technology to exploit the user’s own nerves and muscles to assist with movement and control of the exoskeleton—even for patients with complete paraplegia. Coordinating nerves and muscles with motion can also be beneficial for people undergoing physical rehabilitation for spinal cord injuries or stroke, where walking with the support and assistance of an exoskeleton can significantly improve recovery, even if the exoskeleton does most of the work.

“You want to train up that neurocircuitry again, that firing of patterns that results in locomotion in the corresponding muscles,” explains Ames. “And the only way to do that is have the user moving dynamically like they would if they weren’t injured.”

Caltech is partnering with a French company called Wandercraft to transfer this research to a clinical setting. Wandercraft has developed an exoskeleton that has received clinical approval in Europe, where it has already enabled more than 20 paraplegic patients to walk. In 2020, the RoAMS initiative will focus on directly coupling brain or spine interfaces with Wandercraft’s exoskeleton to achieve stable dynamic walking with integrated neurocontrol, which has never been done before.

Ames notes that these exoskeletons are designed to meet very specific challenges. For now, their complexity and cost will likely make them impractical for most people with disabilities to use, especially when motorized wheelchairs can more affordably fulfill many of the same functions. But he is hoping that the RoAMS initiative is the first step toward bringing the technology to everyone who needs it, providing an option for situations that a wheelchair or walker can’t easily handle.

“That’s really what RoAMS is about,” Ames says. “I think this is something where we can make a potentially life-changing difference for people in the not-too-distant future.”

This article appears in the January 2020 print issue as “This Exoskeleton Will Obey Your Brain.”

How Chicken Beat Beef in America

Par Vaclav Smil
Photo of a chicken standing on a stack of three books.
Photo: Tamara Staples/Getty Images

For generations, beef was the United States’ dominant meat, followed by pork. When annual beef consumption peaked in 1976 at about 40 kilograms (boneless weight) per capita, it accounted for nearly half of all meat. Chicken had just a 20 percent share. But chicken caught up by 2010, and in 2018 chicken’s share came to 36 percent of the total, nearly 20 percentage points higher than beef. The average American now eats 30 kg of boneless chicken every year, bought overwhelmingly as cut-up or processed parts (from boneless breast to Chicken McNuggets).

The United States’ constant obsession with diet, in this case the fear of dietary cholesterol and saturated fat in red meat, has been a factor in the shift. The differences, however, are not ­striking: 100 grams of lean beef has 1.5 grams of saturated fat, compared with 1 gram in skinless chicken breast—which actually has more cholesterol. But the main reason for chicken’s ascendance has been its lower price, which reflects its metabolic advantage: No other domesticated land animal can convert feed to meat as efficiently as broilers. Modern breeding advances have had a lot to do with this efficiency.

During the 1930s, the average feeding efficiency for broilers (at about 5 units of feed per unit of live weight) was no better than for pigs. That rate was halved by the mid-1980s, and the latest U.S. Department of Agriculture’s feed-to-meat ratios show that it now takes only about 1.7 units of feed (standardized in terms of feed corn) to produce a unit of broiler live weight, compared with nearly 5 units of feed for hogs and almost 12 units for cattle.

Because edible weight as a share of live weight differs substantially among the leading meat species (about 60 percent for chicken, 53 percent for pork, and only about 40 percent for beef), recalculations in terms of feeding efficiencies per unit of edible meat are even more revealing. Recent ratios have been 3 to 4 units of feed per unit of edible meat for broilers, 9 to 10 for pork, and 20 to 30 for beef. These ratios correspond to average feed-to-meat conversion efficiencies of, respectively, 15, 10, and 4 percent.

In addition, broilers have been bred to mature faster and to put on an unprecedented amount of weight. Traditional free-running birds were slaughtered at the age of one year, when they weighed only about 1 kg. The average weight of American broilers rose from 1.1 kg in 1925 to nearly 2.7 kg in 2018, while the typical feeding span was cut from 112 days in 1925 to just 47 days in 2018.

Chicken vs. Beef: The dominance of chicken over beef stems in part from a change in consumers’ preferences, itself the result of evolving health guidelines. Even more important is the drop in the relative cost of chicken due to greatly improved efficiency in the conversion of feed into meat.

Consumers benefit while the birds suffer. They gain weight so rapidly because they can eat as much as they want while being kept in darkness and in strict confinement. Because consumers prefer lean breast meat, the selection for excessively large breasts shifts the bird’s center of gravity forward, impairs its natural movement, and puts stress on its legs and heart. But the bird cannot move anyway: According to the National Chicken Council, a broiler is allotted just 560 to 650 square centimeters, an area only slightly larger than a sheet of standard A4 paper. As long periods of darkness improve growth, broilers mature under light intensities resembling twilight. This condition disrupts their normal circadian and behavioral rhythms.

On one side, you have shortened lives (less than seven weeks for a bird whose normal life span is up to eight years) with malformed bodies in dark confinement; on the other, in late 2019 you got retail prices of about US $2.94 per pound ($6.47 per kilogram) for boneless breast compared with $4.98/lb. for round beef roast and $8.22/lb. for choice sirloin steak.

But chicken’s rule hasn’t yet gone global: Thanks to its dominance in China and in Europe, pork is still about 10 percent ahead worldwide. Still, broilers mass-produced in confinement will, almost certainly, come out on top within a decade or two.

This article appears in the January 2020 print issue as “Why Chicken Rules.”