Gregor Mendel’s experiments on pea plants, conducted in the 1850s and ’60s, are introduced to a wider audience thanks to three botanists independently studying inheritance.
Gregor Mendel’s experiments on pea plants, conducted in the 1850s and ’60s, are introduced to a wider audience thanks to three botanists independently studying inheritance.
Albert Einstein proposes that light travels through space in the form of particles, later called photons.
Geologist Arthur Holmes publishes The Age of the Earth, presenting the first complete geologic timescale and arguing for using radioactive materials as geologic clocks.
Physicists Lawrence Bragg and William Henry Bragg demonstrate that X-rays can be used to accurately determine the position of atoms in a crystal. X-ray crystallography would prove essential in the discovery of the structure of DNA and many other biological molecules.
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With the 1918 influenza pandemic racing through military training camps, the U.S. Army Nurse Corps began accepting Black nurses two days after World War I ended. Nine of the first group of 18, shown here around 1919, went to Camp Sherman in Ohio.
A deadly strain of influenza spreads around the globe, ultimately killing an estimated tens of millions of people.
Miners in what’s now Zambia unearth a roughly 300,000-year-old humanlike skull. Known as the Kabwe or Broken Hill skull, it’s the first ancient hominin fossil discovered in Africa.
Psychoanalyst Sigmund Freud, shown, describes mental life as a series of conflicts between a person’s primitive instincts, or id, and moral conscience, or superego, mediated by the ego’s considerations of what’s socially acceptable.
Two scientists at Washington University of St. Louis, Edgar Allen and Edward Doisy, first isolated estrogen in experimental mice and found that it was produced in the ovaries.
Russian physiologist Ivan Pawlow (Pavlov) reports that mice learn to associate an electric bell with dinner after 300 lessons of the bell accompanying food.
American physicist Arthur Compton reports that X-rays lose energy when they are scattered by charged particles. The “Compton effect” indicates that light has a particle nature.
The discovery in South Africa of a 2.8-million-year-old skull with a blend of apelike and humanlike traits hints that the earliest phases of human evolution happened in Africa. Raymond Dart, shown with the fossil, called the Taung Child, places it into a new genus: Australopithecus.
French physicist Louis de Broglie introduces the idea that particles, such as electrons, could exhibit the properties of waves.
German physicist Werner Heisenberg develops a way to describe the energies of electrons in atoms using matrix algebra. His approach became known as matrix mechanics.
William D. Harkins achieves transmutation of elements, converting nitrogen to fluorine and then to hydrogen and oxygen by bombarding the starting element with a helium nucleus. A German physicist later hits gold with hydrogen to make mercury.
The Ford Trimotor takes its first flight. The all-metal plane was reliable and comfortable for passengers, and commercial airlines quickly adopted it.
Physicist Erwin Schrödinger develops “wave mechanics,” a way to describe the energies of electrons in atoms by viewing electrons as waves. Wave mechanics was soon shown to be mathematically equivalent to the matrix mechanics proposed by Werner Heisenberg the previous year.
German physicist Max Born shows that Erwin Schrödinger’s wave equation can be used to calculate the probabilities for various possible outcomes of an atomic observation but does not allow prediction of a single specific outcome.
Based on a tooth found at China’s Zhoukoudian site, Davidson Black identifies a new hominin species named Sinanthropus pekinensis (now called Homo erectus) that lived several hundred thousand years ago.
Danish physicist Niels Bohr presents the principle of complementarity, arguing that both particle and wave views are necessary for a full description of the subatomic world.
German physicist Werner Heisenberg (shown) deduces that it is impossible to precisely measure both the location and velocity of a subatomic particle at the same time. Heisenberg’s principle is called both revolutionary and disturbing by Science News-Letter in 1929.
Edwin Hubble (shown) reports that distant galaxies appear to be flying away from us faster than nearby galaxies, crucial evidence that the universe is expanding.
Ronald Fisher publishes a mathematical analysis of how natural selection can change the distribution of genes in a population, helping to synthesize Mendelian genetics with Darwin’s theory of evolution.
Sewall Wright begins to publish work showing that “random drift,” or chance fluctuations in a population’s gene frequencies, could be a significant factor in evolution.
Heavy hydrogen atoms, now known as deuterium, are discovered by Harold Urey and George M. Murphy.
In his book The Moral Judgment of the Child, developmental psychologist Jean Piaget argues that children develop moral ideas in three broad stages largely via interactions with peers.
Pioneering memory researcher Frederic Bartlett publishes experiments showing that what people remember about past events consists of a mix of fact and culturally influenced fill-ins.
British researchers isolate the virus that causes influenza for the first time, after using human throat washings to infect ferrets.
The Richter scale is proposed by seismologist Charles Richter (shown) to compare the magnitude of different earthquakes. The more accurate moment magnitude scale is now used for most earthquakes.
A new subatomic particle somewhere between an electron and a proton in mass, later termed the muon, is reported from debris of cosmic ray bombardments.
Eugene Houdry’s process for using aluminum- and silicon-based catalysts to make gasoline is demonstrated at an industrial scale.
In South Africa, Robert Broom identifies a robust form of hominin, named Paranthropus robustus, that had giant molar teeth and a skull built for heavy chewing. The species, now known to have lived 1.8 million to 1.2 million years ago, is evidence that more than one type of hominin once called the region home.
Claude Shannon’s master’s thesis “A Symbolic Analysis of Relay and Switching Circuits,” published in 1938, outlines the foundations for digital circuit design.
Engineer and amateur meteorologist Guy Stewart Callendar links a rise in average temperatures around the globe to the release of carbon dioxide from the burning of fossil fuels.
Chemist Roy Plunkett invents the superslick material polytetrafluoroethylene, more commonly known by its trademark Teflon.
In a discovery that portends the possibility of atomic bombs, Otto Hahn (right) and Fritz Strassmann report evidence that uranium atoms produce barium when bombarded with neutrons. As explained by their collaborator Lise Meitner (left) and her nephew Otto Frisch, this is fission, the splitting of atoms.
After creating large amounts of carbon-14 using a cyclotron, or “atom-smasher,” researchers hail the isotope’s potential as a medical tracer. In 1949, Willard Libby and colleagues demonstrate that radioactive carbon can be used to measure the age of organic objects like fossils. The technique works on samples up to about 50,000 years old.
Physicists in the United States produce the first controlled, self-sustaining nuclear chain reaction, in a major step toward an atomic bomb.
With the formation of the Manhattan Engineer District, the United States kick-starts a massive project to build an atomic bomb, which employed more than 120,000 people at its peak.
DDT first arrives in the United States (1945 treatment shown), with immediate implications for curbing the spread of insect-borne diseases. In 1972, amid concerns of its toxicity to humans and other animals, William Ruckelshaus, Environmental Protection Agency administrator, announces the cancellation of all uses of DDT in the United States.
Leo Kanner reports on his studies of a mental illness in 11 children that causes them to largely ignore the people around them, a disease that would come to be called autism.
Oswald Avery, Colin MacLeod and Maclyn McCarty report evidence that DNA is the carrier of genetic information, though the result is not widely accepted at first.
Experiments in mice show that chemicals can — like radiation — induce mutations.
Physicist Walter M. Elsasser and geophysicist Edward Bullard separately propose between 1946 and 1949 that Earth’s magnetic field is a self-sustaining dynamo generated by the movement of fluid in the liquid outer core, which produces an electric current.
The University of Pennsylvania rolls out the first all-electronic general-purpose digital computer, called ENIAC (one shown). The Colossus electronic computers had been used by British code-breakers during World War II.
J. Lawrence Pool is the first to implant electrodes into a woman with Parkinson’s disease.
Claude Shannon’s “A Mathematical Theory of Communication” establishes the new field of information theory.
Canadian psychologist Donald Hebb publishes his account of how neurons reinforce each other’s activity, a description that’s now shorthanded, “fire together, wire together.”
A team led by meteorologist Jule Charney produces the first computer-driven weather forecast.
William W. Morgan and colleagues present a model of the Milky Way’s shape made of cotton balls to the American Astronomical Society meeting, depicting the galaxy as a spiral. The team received a raucous ovation with stomping feet.
Luis Miramontes (shown) made one of the first active ingredients in birth control pills – norethindrone.
Phillips Petroleum chemists J. Paul Hogan and Robert Banks, while studying new ways to make gasoline, discover polypropylene, which also leads to a new way to make high-density polyethylene.
A blender experiment by Alfred Hershey and Martha Chase demonstrates that DNA, not protein, is the genetic material.
Building on earlier work by French physicist Louis de Broglie, theoretical physicist David Bohm suggests a deterministic interpretation of quantum theory that incorporates “hidden variables.”
A patient known as H.M. undergoes surgery to remove his hippocampus, later revealing the role that the brain structure plays in memory. Studies of his postmortem brain (shown) complicated the picture.
James Watson (left) and Francis Crick (right) report in Nature the discovery of the double-helix structure of the DNA molecule. Papers from Rosalind Franklin and Maurice Wilkins, which appear alongside Watson and Crick’s report, provide essential evidence.
Vincent G. Allfrey, Alfred E. Mirsky and Marie Maynard Daly (shown) report direct experimental evidence that protein synthesis requires RNA.
Narinder Singh Kapany successfully sent high-quality images through a bundle of optical fibers.
Marguerite Vogt and Renato Dulbecco publish a method for purifying and counting poliovirus particles. Vogt and Dulbecco’s approach remains the gold standard for purifying and counting virus particles.
Psychologist George Miller reports that people can typically hold no more than about seven types or pieces of information in short-term memory, a capacity Miller dubbed “the magical number seven, plus or minus two.” He publishes the work the next year.
A workshop for a small group of scientists organized by John McCarthy at Dartmouth College is often considered the beginning of the field of artificial intelligence.
Meteorologist Norman Phillips develops the world’s first general circulation climate model, which captures how energy flows between the oceans, atmosphere and land.
The first full-scale commercial nuclear power plant, known as Calder Hall, switched on in Cumbria, England.
A decision-making model developed by economist Herbert Simon contends that people use experience-based rules of thumb to work around limited knowledge and time when dealing with complex challenges, such as playing chess.
Columbia University researchers Bruce Heezen, Marie Tharp (shown) and Maurice Ewing create the first comprehensive map of an ocean basin, revealing a deep rift right at the center of a long underwater mountain chain cutting through the North Atlantic.
Scientists Roger Revelle and Hans Suess report that oceans do not take up as much carbon dioxide as previously thought, which suggests much of the gas produced by human activities is going into the atmosphere.
John Bardeen, Leon Cooper and Robert Schrieffer set out a theory explaining how electron pairs can flow without resistance through low-temperature materials.
American physicist Hugh Everett III proposes what’s now known as the “many worlds” interpretation of quantum mechanics. An experiment does not create one reality from many quantum possibilities, argues Everett (shown), but instead identifies only one of many branches of reality.
President Eisenhower signs legislation establishing the National Aeronautics and Space Administration, better known as NASA.
Geochemist Charles David Keeling (shown in 1988) begins tracking the rise in atmospheric carbon dioxide at Mauna Loa in Hawaii. The record, which continues through today, has become one of the most iconic datasets in all of science.
Physicist Roger Bacon, working at Union Carbide in Parma, Ohio, demonstrates the first high-performance carbon fibers.
In a widely cited book review, linguist Noam Chomsky criticizes B.F. Skinner’s view of language as learned behavior and suggests that human speech springs from an innate grammar capacity.
David Hubel and Torsten Wiesel illuminate the visual system in the cat, opening up an area of inquiry for how brain systems handle information from the senses.
Frank Drake aims a radio telescope (the Tatel Telescope in Green Bank, W.Va., shown) at two stars to listen for signs of alien civilizations, the first SETI experiment.
The U.S. Food and Drug Administration approves the first oral contraceptive, soon known simply as “the pill.”
Scientists reveal how the sequence of DNA’s four chemical subunits encode the instructions for the creation of the amino acids that make up proteins. One team including Francis Crick and Sydney Brenner discovers that it’s a triplet code, with each sequence of three subunits, or “genetic letters,” coding for one amino acid. In the same year, Marshall Nirenberg and Heinrich Matthaei report the three-letter sequence associated with one particular amino acid, jump-starting efforts to identify more.
Jacques Monod and François Jacob describe messenger RNA and its role in carrying genetic information from the cell nucleus to the ribosome for protein synthesis.
Geophysicist J. Tuzo Wilson suggests that volcanic island chains form as plates move over upwellings of magma in the mantle.
Louis Leakey and colleagues report that a collection of fossils found at Tanzania’s Olduvai Gorge are the earliest known remains from the genus Homo. The researchers fold the fossils into a new species, Homo habilis, thought to have lived about 2.4 million to 1.4 million years ago. The classification remains controversial.
Marian Diamond provides early evidence for the brain’s ability to change, or plasticity, later summarizing her findings with the phrase, “Use it or lose it.”
Beginner’s All-purpose Symbolic Instruction Code, a computer programming language designed to be easy to use, successfully runs its first programs on a General Electric computer at Dartmouth College.
Physicist John Bell devises a mathematical theorem that would allow researchers to experimentally rule out any “hidden variables” that might explain the results of quantum entanglement experiments in classical, deterministic terms.
Norwegian physiologist Terje Lømo observes that connections between nerve cells strengthen with use, a core principle of neuroscience called long-term potentiation.
Harry Whittington (shown) leads an expedition to Canada’s Burgess Shale, identifying a riot of new and unusual forms of animal life and boosting studies into the Cambrian explosion.
By studying blood proteins, Vincent Sarich and Allan Wilson develop a “molecular clock” for primate evolution. The pair estimates that humans and African apes diverged about 5 million years ago. The latest estimates suggest that humans and chimpanzees, now known to be our closest living relative, diverged sometime between 9 million and 6 million years ago.
Meteorologists Syukuro Manabe and Richard Wetherald model connections between Earth’s surface and atmosphere and calculate how changes in carbon dioxide would affect the planet’s temperature.
The first mumps vaccine, developed by Maurice Hilleman, is licensed in the United States.
American inventor Douglas Engelbart, at a meeting in San Francisco, demonstrates in one system most of the elements of modern personal computing.
Robert Heath Dennard receives a patent for dynamic random access memory that requires just one transistor, ultimately leading to big boosts in computer memory density.
A book by biologists Paul and Anne Ehrlich popularizes the notion of a “population bomb” in which global overpopulation would lead to mass starvation.
The ARPANET is born, as two computers — one at UCLA and one at the Stanford Research Institute — link up and share a message.
The first Earth Day, organized by U.S. senator Gaylord Nelson and graduate student Denis Hayes, is celebrated.
Two research teams working independently discover that tumor viruses can use an enzyme, called reverse transcriptase, to transfer genetic information from their RNA into the DNA of a host cell.
Federico Faggin, Ted Hoff and Stanley Mazor develop what’s widely considered the first microprocessor, the Intel 4004, a single-chip central processing unit, or CPU, with 2,300 transistors.
A group of women in India lead a series of widely publicized protests against deforestation, linking environmental protection with the protection of human communities.
At the site of Hadar in Ethiopia, Donald Johanson (shown) and Tom Gray discover a nearly 40 percent complete hominin skeleton dating to 3.2 million years ago. Widely known as Lucy, the fossil is later classified as Australopithecus afarensis and claimed to be a direct ancestor of the genus Homo.
Two teams of physicists find a new subatomic particle, the J/psi, which, it soon became clear, could be explained only by a new type of quark, the charm quark. This discovery and others during the period, known as the November revolution, quelled lingering doubts that quarks were real constituents of larger particles such as protons, neutrons and the J/psi.
Researchers report evidence that Freon and other chlorofluorocarbons destroy stratospheric ozone.
Astronomers William Hartmann and Donald Davis propose in 1975 the Giant Impact Hypothesis, suggesting the moon formed out of material ejected from Earth after a large collision. Space scientist Alastair Cameron and astronomer William Ward developed the same idea independently, proposing it in 1976.
George Köhler and César Milstein report finding a technique for merging cells that produce a particular antibody with tumor cells to grow vast quantities of descendant cells that produce the original, single antibody.
The first spacecraft to survive on Mars for an extended time looked for signs of life, but its results were inconclusive.
Fossilized footprints at the Tanzanian site of Laetoli (shown) reveal that by 3.6 million years ago hominins were already competent upright walkers.
Scientists find that Earth’s ice ages over the last 500,000 years correlate to three different orbital variations — cycles lasting about 23,000 years, 42,000 years (now said to be 41,000) and 100,000 years, confirming a hypothesis proposed decades earlier by astronomer Milutin Milankovitch.
Carl Woese and George Fox report identifying the third domain of life, adding the archaebacteria (now commonly called Archaea) alongside bacteria and eukaryotes.
Three computers released this year — the Commodore PET, the Apple II and the TRS-80 (an early version shown) — help make personal computing a reality.
Psychologist Francine Patterson reports that Koko, a “talking” gorilla, has a sign language vocabulary of 375 words (Patterson and Koko are shown). Two chimps exhibit “the first instance of symbolic communication between nonhuman primates.”
Work reported by Edward Lewis in 1978 and Christiane Nüsslein-Volhard and Eric Wieschaus in 1980 reveals which genes control early embryonic development in fruit flies. The discoveries help explain congenital defects and lead to research on similar genes that determine the human body plan.
Luis Alvarez and Walter Alvarez publish a report saying that a large space rock hitting Earth was responsible for the mass extinction event 66 million years ago that killed off all nonbird dinosaurs. The father-son team proposed the idea at American Geophysical Union’s annual meeting the year before.
In a big switch, the American Psychiatric Association issues a diagnostic manual that mostly drops psychoanalytic terms and uses sets of symptoms to define mental disorders.
An outbreak of two rare and serious diseases among gay men — Kaposi’s sarcoma and Pneumocystis pneumonia — mark the discovery of what would come to be known as AIDS.
NASA reports satellite evidence that the stratospheric ozone layer is being depleted globally.
Following earlier experiments in the 1970s, French physicist Alain Aspect and colleagues report the strongest test to date of entanglement. The results agree with quantum mechanical predictions and show no signs of hidden variables.
Françoise Barré-Sinoussi and Luc Montagnier discover the retrovirus that is eventually named Human Immunodeficiency Virus, the cause of AIDS.
Chuck Hull invents stereolithography, an early 3-D printing technique.
George Glenner and Caine Wong discovered a “novel cerebrovascular amyloid protein.” That was amyloid-beta, the key component of plaques in the brains of people with Alzheimer’s disease.
Physicist Charles Bennett and computer scientist Gilles Brassard propose a theoretical system for quantum cryptography, which would use photons in a superposition of states to create a secure key.
Scientists trace the wiring diagram of the nervous system of a C. elegans worm.
Margaret Geller, John Huchra and Valérie de Lapparent map a section of the observable universe, revealing a structure that encompasses large walls and giant voids.
Physicists Georg Bednorz and Alex Müller discover the first materials that superconduct at relatively high temperatures, kicking off a search for more, similar materials.
The drug fluoxetine, or Prozac, received approval from the U.S. Food and Drug Administration as a treatment for major depression.
Intelligence researcher James Flynn reports large jumps in IQ scores from one generation to the next in many Western nations, indicating that environment shapes performance on intelligence tests.
NASA scientist James Hansen testifies to lawmakers about the consequences of global warming, vaulting climate change into the public eye in the United States.
Alan Handyside of London’s Hammersmith Hospital is able to check an embryo for genetic defects before implanting it into a mother’s uterus. This testing, known as preimplantation genetic diagnosis or PGD, enables parents carrying a genetic or chromosomal defect to avoid passing it to their children.
Based on just nine minutes of data, the Cosmic Background Explorer, or COBE, reveals that the cosmic microwave background radiation aligns with what is expected from blackbody radiation, good evidence that it is an afterglow of the Big Bang.
In the first federally approved gene therapy trial, researchers inject genetically engineered cells into a 4-year-old patient with an inherited immune deficiency.
Delegates at the First National People of Color Environmental Leadership Summit in Washington, D.C., draft and adopt 17 principles of environmental justice, setting a foundation for a growing movement.
Sony releases the first commercial lithium-ion battery (a more modern battery is shown), clearing the way for abundant portable electronics.
World leaders gathered (shown) at the United Nations Conference on Environment and Development in Rio de Janeiro to address how to pursue economic development while also protecting the Earth. The meeting resulted in an international convention on climate change.
An academic battle breaks out among evolutionary psychologists and others over the nature of human decision making and what it means to be rational.
The Intergovernmental Panel on Climate Change establishes that there is evidence of a “discernible” human influence on climate.
Dolly the Sheep (shown) is the first mammal cloned from the DNA of an adult mammal. Ian Wilmut and Keith Campbell of the Roslin Institute transferred the nucleus of an adult mammary gland cell into an egg cell, showing that adult DNA can be reprogrammed to grow a new organism.
Physicists discover that neutrinos, long thought to be massless, have a tiny amount of mass. The find raises a slew of still-unanswered questions, whether neutrinos are their own antiparticles, for example, or whether the particles can help explain the scarcity of antimatter in the universe.
Australian researcher Alan Trounson reports successful vitrification, a method that freezes eggs so rapidly that no crystals can form. The technique boosts the success rate for using frozen eggs in assisted reproduction.
Neurologist Helen Mayberg begins work testing deep brain stimulation on people with severe depression, targeting an area called the subcallosal cingulate.
Physicists at Brookhaven National Laboratory smash nuclei of gold atoms together to recreate the phase of matter thought to have existed in the early moments of the universe. The quark-gluon plasma is a hot, dense soup of quarks intermingling freely with gluons, which under normal conditions bind quarks into larger particles like protons and neutrons.
China passes the United States as the world’s biggest emitter of carbon dioxide, as measured by annual emissions.
The Kepler space telescope (illustrated) found thousands of planets outside our solar system before it stopped operating in 2018.
Researchers get a surprise when mitochondrial DNA extracted from a stray finger bone from Russia’s Denisova Cave matches neither Neandertal nor modern human DNA, suggesting a previously unknown hominin lived there.
Cross-cultural researchers led by anthropologist Joseph Henrich review evidence that members of Western, educated, industrialized, rich and democratic (WEIRD) societies are among the least useful populations to study in order to generalize about how resources get shared and other features of human nature.
North Carolina sediment cores reveal that sea levels began rising precipitously in the late 19th century, a trend attributed to climate change.
Long-standing inabilities to confirm influential psychology findings in repeat studies spark an ongoing debate over research methods and practices.
A paralyzed woman controls a robotic arm with her mind, enabling her to drink coffee from a bottle.
What’s known as a convolutional neural network makes a big leap in performance in the annual ImageNet Challenge, an object detection and image classification competition. Convolutional neural networks would become the standard architecture for computer vision.
Astronomers report detecting ripples in spacetime imprinted on the cosmic microwave background radiation, the flash of light released into space about 380,000 years after the Big Bang. It would have been the first direct evidence of cosmological inflation, but it turned out not to be true.
An Ebola outbreak in West Africa that began in 2014 and continued through 2016 is the largest to date (health workers assisting a patient in Guinea are shown). The second largest outbreak, in the Democratic Republic of the Congo, begins in 2018 and ends in 2020.
Lee Berger and colleagues report finding a small-brained species of Homo, named Homo naledi, in a South African cave. Although the team thinks H. naledi was an early member of the genus, later dating shows it lived only about 300,000 years ago, making it a contemporary of Homo sapiens.
The first baby with three biological parents is born with DNA from his mother, father and the mitochondria of an egg donor. The birth raises hopes for preventing inheritance of mitochondrial disorders —and triggers fears of “designer babies.”
The Keeling curve, an iconic graph that tracks the rise in atmospheric carbon dioxide, passes an annual minimum of 400 parts per million.
Jean-Jacques Hublin and colleagues announce that excavations in Morocco have uncovered Homo sapiens fossils dating to about 300,000 years ago. The discoverers say the remains are the oldest known members of our species.
Chinese scientist Jiankui He uses CRISPR/Cas9 to create the first gene-edited babies — twin girls — to reduce their risk of contracting HIV. His actions lead to the formation of an international commission on the clinical use of gene editing.
Investigators report using a public genealogy database to track down a man suspected of being the Golden State Killer, prompting a flurry of concerns about privacy and ethics. The man, Joseph James DeAngelo, pleaded guilty to 13 murders and 13 rape-related charges.
Activist Greta Thunberg initiates the “School Strike for Climate” movement by protesting outside the Swedish parliament. Soon, students around the world join a growing movement demanding action on climate change. (Activists at the 2021 U.N. Climate Change Conference are shown.)
The gene editor CRISPR/Cas9 (illustrated) enters its first clinical trials, to combat cancer, blindness and blood disorders in people.
SpaceX launches three astronauts to the International Space Station, marking the first time a private company sent humans into orbit.
A National Academies of Sciences, Engineering and Medicine report documents rising numbers of premature deaths in the United States since the 1990s from drug overdoses, alcohol abuse, suicides and obesity. COVID-19 exacerbated that trend, the report concludes.