AEvery day, the average person makes thousands of decisions. Most are trivial — what to eat for breakfast, which route to take to work, how to respond to a casual email. Yet research in cognitive psychology suggests that each decision, however minor, draws on a finite reservoir of mental energy. When that reservoir runs low, the quality of decision-making deteriorates in predictable and sometimes consequential ways. This phenomenon, known as decision fatigue, has implications ranging from courtroom rulings to supermarket shopping habits.

BThe concept emerged from research conducted by social psychologist Roy Baumeister in the late 1990s, who proposed what he called ego depletion — the idea that self-regulation and decision-making rely on a limited resource, analogous to a muscle that tires with use. Baumeister's laboratory studies demonstrated that participants who were required to make a series of choices performed significantly worse on subsequent tasks requiring willpower or cognitive effort. The theory attracted considerable attention and spawned a large body of supporting research.

CPerhaps the most striking real-world demonstration of decision fatigue came from a 2011 study of Israeli parole board hearings by Shai Danziger and colleagues. The study examined more than 1,000 rulings made over ten months and found that the probability of a favourable ruling — granting parole — was highest at the start of the day and immediately after meal breaks, falling sharply as the session progressed. By the end of a session, the likelihood of parole being granted dropped to nearly zero. The researchers interpreted this as evidence that mental depletion leads judges to default to the safest option — denial — rather than engaging in the complex deliberation required to justify release.

DDecision fatigue also affects consumer behaviour in ways that retailers have long understood, even if they lacked the scientific vocabulary to describe it. Supermarkets strategically place impulse purchase items near checkouts, knowing that shoppers who have spent an hour navigating aisles and making choices are in a depleted state. Similarly, car dealerships structure the customisation process — beginning with the most significant decisions before moving to less costly options — in ways that leave buyers tired and less likely to negotiate robustly at the end.

EThe implications for personal productivity have also attracted considerable interest. Many highly productive individuals, including former US President Barack Obama and the late Apple co-founder Steve Jobs, reportedly adopted strategies to minimise decision fatigue. Obama famously wore only grey or blue suits, citing the cognitive cost of choosing clothing as a reason for standardising his wardrobe. These approaches align with a broader body of research suggesting that structuring routines to reduce unnecessary choices preserves mental energy for more important decisions.

FHowever, the scientific consensus on ego depletion has been challenged in recent years. A large-scale replication effort involving researchers at multiple institutions, published in 2016, failed to reproduce many of Baumeister's original findings. Critics have argued that the original studies were underpowered and that the effect may be considerably smaller than initially reported, or contingent on factors such as participants' beliefs about willpower itself. Research by Carol Dweck and colleagues suggested that ego depletion effects were largely absent in participants who did not believe willpower to be a limited resource.

GThe parole board study, too, has been reinterpreted. Later analyses pointed out that the timing of breaks correlated with the types of cases heard, raising the possibility that the apparent fatigue effect was partially explained by case complexity rather than mental depletion alone. Despite these critiques, most researchers agree that decision-making quality does decline under certain conditions of mental load and that environmental design — structuring when and how decisions are made — can have meaningful effects on outcomes.

HFor individuals, the practical takeaway is relatively straightforward: tackle the most important decisions when mental resources are freshest, typically in the morning or after rest. Reducing the number of low-stakes decisions through routines and pre-commitment strategies can also help. For institutions — whether courts, hospitals, or financial firms — the research raises important questions about the conditions under which high-stakes decisions are made, and whether systems can be designed to mitigate the effects of cognitive depletion.

AThe deep ocean — broadly defined as waters below 200 metres, where sunlight no longer penetrates — covers more than 60 percent of Earth's surface and remains one of the least explored environments on the planet. For much of scientific history, this vast, cold realm was assumed to be largely devoid of life. The discovery of hydrothermal vents in 1977 fundamentally overturned that assumption, revealing ecosystems of astonishing complexity sustained entirely without sunlight. At the heart of these ecosystems lie some of the most extraordinary symbiotic relationships in biology.

BHydrothermal vents form along mid-ocean ridges, where tectonic plates diverge and magma heats seawater to temperatures exceeding 400 degrees Celsius. As superheated water is expelled through chimney-like structures known as black smokers, it carries dissolved minerals and chemicals — including hydrogen sulphide — that would be toxic to most surface life. Yet around these vents, communities of organisms thrive in densities comparable to tropical rainforests, sustained by a process called chemosynthesis. Unlike photosynthesis, which uses sunlight to convert carbon dioxide into organic matter, chemosynthesis uses chemical energy — derived from the oxidation of hydrogen sulphide and other compounds — to fuel the production of organic molecules.

CThe primary producers in vent ecosystems are chemosynthetic bacteria and archaea. These microorganisms form the base of a food web that supports a remarkable array of animals: tube worms that can reach two metres in length, dense clusters of mussels and clams, ghostly white crabs, and fish adapted to extreme pressure and darkness. Many of these animals cannot synthesise the nutrients they require independently and have evolved intimate relationships with chemosynthetic microbes.

DAmong the most thoroughly studied of these relationships is that between the giant tube worm (Riftia pachyptila) and its internal bacterial symbionts. Adult tube worms lack a digestive tract entirely. Instead, they possess a specialised organ called the trophosome, which is densely packed with chemosynthetic bacteria. The worm absorbs hydrogen sulphide, oxygen, and carbon dioxide through its plume — the feathery red structure visible at the tube's opening — and transports these compounds to the bacteria within the trophosome via specialised haemoglobin molecules. The bacteria use these raw materials to produce organic compounds that nourish the worm. It is an arrangement of extraordinary mutual dependence: the bacteria gain a protected, chemically rich environment, while the worm obtains its entire nutritional requirement from its internal microbial partners.

ESimilar endosymbiotic arrangements occur in the large clams and mussels found at hydrothermal vents and cold seeps — areas of the seafloor where hydrocarbon-rich fluid seeps from the sediment. Clams in the genus Calyptogena house chemosynthetic bacteria within the cells of their gills. Remarkably, the clams actively position themselves at the boundary between oxygen-rich water and sulphide-rich sediment, using their muscular foot to bridge this chemical divide and supply both compounds to their gill bacteria simultaneously.

FBeyond endosymbiosis, vent ecosystems display other symbiotic arrangements. The Pompeii worm (Alvinella pompejana), found at some of the hottest vent sites, lives within tubes attached directly to vent chimneys and is covered in a thick mat of filamentous bacteria on its dorsal surface. Researchers hypothesise that these bacteria may form an insulating layer that helps the worm tolerate the extreme thermal gradient between the scalding chimney surface and the relatively cooler surrounding water. However, the exact nature of the relationship — whether it is mutualistic, commensal, or parasitic — remains a subject of active investigation.

GThe discovery and study of deep-sea vent ecosystems has had far-reaching implications beyond marine biology. The realisation that life can thrive in the complete absence of sunlight, sustained by chemical energy alone, has profound consequences for the search for life beyond Earth. Scientists now consider environments analogous to hydrothermal vents as plausible candidates for extraterrestrial life, particularly on icy moons such as Europa and Enceladus, which are thought to harbour liquid oceans beneath their frozen surfaces. The study of symbiosis in the deep ocean thus extends our conception of the possible conditions for life across the universe.