Why do two individuals faced with the same information sometimes make completely different decisions?
It is a question made famous by Nobel laureate Daniel Kahneman, whose book “Thinking, Fast and Slow” explored the contrast between rapid, intuitive decisions and slower, more deliberative thinking in humans.
Now researchers studying wild fish in Lake Tanganyika have found a striking parallel.
In more than 5,000 underwater trials, researchers from the Max Planck Institute of Animal Behavior (MPI-AB), the Université Clermont Auvergne (UCA) and the French National Centre for Scientific Research (CNRS), tested the decision-making of two closely related species of Featherfin cichlids. When decisions were simple, the fish behaved identically. But when choices became more complex, one species made rapid, decisive choices while the other slowed down and appeared to integrate several pieces of information before deciding.
The findings suggest that evolution or life history can alter not just what animals choose, but how they make decisions.
The researchers studied male Aulonocranus dewindti , the “golden featherfin”, and Cyathopharynx furcifer , the “blue featherfin”, two cichlid species that build elaborate sand bowers to attract females. Males are meticulous housekeepers, immediately removing any foreign object that lands inside their bowers. That natural behavior allowed the researchers to study decision-making in the wild by placing carefully designed 3D-printed objects into the bowers and recording which objects the fish removed first.
First, the team established that the two species shared the same basic preferences. Given simple choices involving color or size, they behaved identically. For instance, they always preferred to remove larger objects first. The two species also preferentially removed an object with a color that stood out from a group, a cognitive bias suggesting that their attention was guided similarly in simple situations. This cognitive bias—known as the “oddity effect”—is also shared with humans, the authors note.
Then the researchers made the choices more difficult by forcing the fish to trade off different features. One object might have the preferred color but be smaller, while another was larger but had a less attractive color.
This time, the species diverged.
The blue featherfin continued to make rapid, decisive choices, based on one feature only. The golden featherfin, meanwhile, did not show a preference between the alternatives and took significantly longer to decide. Most strikingly, increasing the complexity of the task made golden featherfins slower to reach a decision, while blue featherfins actually became faster.
“It was crucial to study this in two well-matched species,” says first author Dr. Maëlan Tomasek , who conducted the research for his doctoral thesis with UCA and MPI-AB. “They could see the same things, they were equally motivated, and they agreed when the choices were simple. Only when we made the decisions more demanding did they suddenly diverge. To be honest, that was a surprise. Why would two species so closely related behave so differently in complex situations?”
The contrast echoes ideas from human psychology, but the researchers stress that they are not claiming fish possess human-like versions of Kahneman’s “fast” and “slow” thinking.
“The comparison is compelling because the underlying problem is the same,” says co-author Dylan Naceur , a doctoral student in cognitive psychology at UCA, whose research focuses on human attention. “Every brain has limited attentional resources, so it must decide which information deserves priority. Humans and fish might not think in the same way, but both face the same computational challenge of selecting the most relevant information from a complex world.”
To test whether the slower species was simply becoming confused, the researchers borrowed another classic experiment from human decision science: the decoy effect task. The decoy effect is a psychological phenomenon in which people change their preference between two options after a third, inferior option is introduced. This cognitive bias is often used in marketing to influence human choices.
When the fish were presented with the decoy effect task, the golden featherfin continued to treat the two main alternatives as equally attractive while largely ignoring the inferior option. “It was genuinely considering multiple features rather than choosing randomly, and it was making rational decisions, which many humans fail to do when faced with the same task,” adds Tomasek, now a postdoctoral researcher at Monash University in Australia.
The results lay bare the diverse cognitive strategies used by animals, even of closely related species; yet the authors caution how we interpret these differences. “Comparative cognition has often focused on ranking species by their cognitive aptitude, their ‘cleverness’,” says co-senior author Dr. Valérie Dufour , a research director in social and cognitive psychology at UCA and CNRS. “Our results suggest a different perspective. Closely related species may solve the exact same problem using different cognitive strategies, but one strategy is not necessarily better than the other. Sometimes, deciding faster is better. Sometimes, considering all information is better. Understanding those strategies may be more important than attempting to measure abstract intelligence itself.”
More research is needed to determine the source of the divergence between the species. The difference in decision-making could indeed arise through different life experiences, but also through evolutionary divergence. If the second alternative turns out to be true, the authors claim that this would tell us a lot about the evolution of intelligence.
“Evolution does not cause animals to come up with more and more complex decision processes,” says co-senior author Dr. Alex Jordan , an independent group leader at MPI-AB. “Rather, it increases their fit to the environments and challenges they face, meaning that when simple solutions work, they need not be replaced by more complicated rules. Here we have two solutions that appear to work for different reasons. One integrates broadly before deciding, while the other rapidly focuses on what matters most. They start out with the same capacities, but it appears evolution has changed the decision rule to solve similar problems in different ways.” Why that might be and what the benefits of each strategy are remains a question for future work, the authors say.
The work also demonstrates that sophisticated cognitive experiments, originally developed for human psychology, can be carried out on wild animals in their natural environment. Rather than training captive animals to perform artificial tasks, the researchers exploited a behavior the fish perform naturally every day: keeping their bowers clean.
The authors hope the study encourages closer links between psychology, evolutionary biology and animal cognition, using natural systems to understand not simply whether animals think, but how they do it.
Proceedings of the National Academy of Sciences
Experimental study
Animals
Decision rules diverge between sympatric Featherfin cichlids despite shared preferences, ecology, and evolutionary history
12-Aug-2026