In the study of decision-making, the well-know speed-accuracy tradeoff is arguably one of the most fundamental phenomena1–4. With more time we make fewer errors. This is true not only for humans across a wide variety of task domains5–8, but even for bees and ants9–10. In perception and memory tasks as well as economical choices, it has been shown that decision-makers can adapt to time pressure, but also strategically set the amount of time and effort they invest in processing information for decision-making, by weighing the cost of time versus the gain in accuracy. Thus, decision-makers can manipulate the speed-accuracy tradeoff, aiming for optimality in given circumstances11–13.
When measuring the accuracy of performance during perceptual discrimination or memory, researchers rely on objective information that can readily be verified. Responses are true or false, whether we like it or not. However, in other domains we often make decisions or judgements based on subjective evaluations that cannot readily be measured in terms of accuracy14–16. One person’s judgment may be wholly different from another’s, yet neither is necessarily wrong. The discourse between tea and wine has been going on for more than a millennium17, and according to the Latin proverb De gustibus non est disputandum (“In matters of taste there can be no dispute”) we are fully entitled to have our own preferences, no matter what everyone else says. Examples of subjective decision-making include those based on preference formation with respect to the perceived attractiveness of faces, foods, or works of art, to name the most commonly researched categories18–20. Another type of subjective decision is based on moral evaluation, which may involve intuition or reasoning, and be shaped by cultural background and personal experience, with a complexity of processing that does not afford an easy metric of accuracy21–23. For these diverse types of subjective decision-making, little is known about the relationship between speed and decision performance.
As a metric of performance in subjective decision-making, the nearest equivalent to accuracy would arguably be the consistency of response, measured across a population of decision-makers (e.g., the level of agreement or consensus) or measured across a sample of decisions for the same items under different conditions (e.g., the level of reproducibility)14. We may have our idiosyncratic preferences, but there should be a system in our proverbial madness. Our tastes would not be random, but follow some kind of internal logic. This logic should give reproduceable answers. Accordingly, one hypothesis here would be that the speed-accuracy tradeoff corresponds to a speed-consistency tradeoff in subjective decision-making. With more time, decision-makers would show higher consistency in their responses. One might argue that, the extra time for information processing should give decision-makers the opportunity to form more precise and more stable object representations of the items under consideration. This would lead to less variability in the evaluation (i.e., a narrower response distribution, with a smaller standard deviation) but not necessarily a change in the mean of the evaluations (i.e., with or without a lateral shift of the response distribution).
On the other hand, speeded responses might be fundamentally different from slow responses – more intuitive, more emotionally charged. Put differently, subjective decision-making might involve a shift in the nature of the underlying evaluation mechanisms as a function of the amount of time available for processing. This would imply an altered relation between speed and performance in subjective decision-making as compared to the speed-accuracy tradeoff in objective decision-making.
Notably, subjective decision-making involves a complex system with parallel and interactive streams of affective processing, including fast, intuitive emotional reactions and slow, controlled evaluations based on reasoning21–22,24−26. By this account, speeded subjective decisions would tend to be more polarized or extreme (highly positive or negative), driven by an emotional charge, whereas slow subjective decisions would be more moderate, controlled by deliberative reasoning that would tend to seek a middle ground or compromise among item characteristics with divergent affective values. Thus, we can raise a speed-polarization hypothesis, proposing that faster responses would lead to more extreme evaluations. Particularly, time pressure would act as a stressor that reinforces the emotional charge and biases the moral evaluation toward fast, intuitive emotional reactions20,27−30. Consequently, urgency would lead to polarization in the evaluation.
In our previous research on preference formation with respect to food images, we found preliminary evidence that at first glance seems to support the speed-polarization hypothesis. In self-paced viewing conditions, fast responses tended to be associated with extreme ratings (both the highest and lowest ratings), whereas slow responses tended to be associated with intermediate ratings16,31−32. Importantly, however, this evidence merely established a correlation between speed and polarization in the preference formation with food images.
In the present study we set out to examine the speed-polarization hypothesis with a different type of subjective decision-making: the moral evaluation of real-world images. We chose to work with moral evaluation for two reasons. First, we aimed to find converging evidence with a very different type of subjective decision for the correlation between speed and polarization. Second, moral evaluation is the type of subjective decision-making whose underlying mechanisms has been characterized in greatest detail, with solid evidence for the existence of fast, emotional processes versus slow, deliberative processes21–22, 24,33–34. Thus, moral evaluation was the appropriate type of decision to examine whether speed actually causes polarization.
We designed the present experimental paradigm to compare the moral evaluation of real-world images under self-paced viewing versus under time pressure (See Figure 1). We prepared a stimulus set ranging from “very moral” to “very immoral” images based on the Socio-Moral Image Database (SMID)35. We asked participants to rate real-world images on a continuous scale from -10 (“very immoral”) to +10 (“very moral”). To create conditions with and without urgency, we used a cueing procedure to instruct the participants at the beginning of each trial whether they could take as long as they wanted to make the evaluation (Self-Paced condition, SP; left panel of Figure 1) or whether they had to make the evaluation within 2 seconds (Time-Limited condition; TL, right panel of Figure 1). In the TL condition, the trial was aborted and an error message was given when the participant took too long to respond.
Figure 2 presents the alternative predictions from two hypotheses. According to the speed-polarization hypothesis (left panel), speed causes the moral evaluations to be more polarized, and so the ratings under time pressure in the TL condition would be more extreme (i.e., more positive ratings for moral images and more negative ratings for immoral images) than the ratings without time pressure in the SP condition. In contrast, the right panel shows an alternative, decision-difficulty hypothesis, by which there would be no differences in the mean ratings between the SP and TL conditions. According to the decision-difficulty hypothesis, speed does not cause polarization. Instead, fast responses may be correlated with extreme ratings as a function of the ease of categorization of moral images. Some images would have a very salient moral content, enabling fast categorization with an extreme rating, whereas other images would have a less salient moral content, requiring more time to categorize and tending to produce more moderate ratings. The decision-difficulty hypothesis implies a correlative relationship, but no causal relationship, between speed and polarization. (Note that the decision-difficulty and the speed-polarization hypotheses are both compatible with the notion of an additional speed-consistency tradeoff, whereby the TL responses should show more variation than the SP responses.)