(A) Experimental paradigm. Each trial consisted of two observation intervals. In each interval, six vertically oriented Gabor patches were displayed equidistantly around an imaginary circle (duration: 85 ms). In either the first or second interval, there was one oddball target that had slightly higher contrast than all the others (in this example, upper left target in interval 1). (B) Two example psychometric functions and the group average in Experiment 1. The proportion of trials in which the oddball was reported to be in the second interval is plotted against the contrast difference at the oddball location (i.e. contrast in the second interval minus contrast in the first). A highly sensitive observer would produce a steeply rising psychometric function with a large slope. Blue circles: performance of the less sensitive observer (*s*_{min}) of the dyad; red squares: performance of the more sensitive observer (*s*_{max}); black diamonds: performance of the dyad (*s*_{dyad}). The blue and red curves are the best fit to a cumulative Gaussian function (); the black curve is the prediction of the weighted confidence-sharing model. (C) The predictions of the four models (see Eqs. -). The *x*-axis shows the ratio of individual sensitivities (*s*_{min}/*s*_{max}), with values near one corresponding to dyad members with similar sensitivity and values near zero to dyad members with very different sensitivity. The *y*-axis shows the ratio of dyad sensitivity to the more sensitive member (*s*_{dyad}/*s*_{max}). Values above the horizontal line indicate communication benefit; in this range the dyad is better than the more sensitive observer. The red curve, which corresponds to the weighted confidence sharing (WCS) model, is above the horizontal line only if *s*_{min}/*s*_{max} is larger than about 0.4, reflecting the prediction that communication by weighted confidence sharing is beneficial only if dyad members have approximately the same competence. The green curve, which corresponds to the direct signal sharing (DSS) model, never crosses the horizontal line, so for this model communication will invariably be beneficial. Dot-dashed and solid black lines indicate the coin flip (CF) and behaviour and feedback (BF) models, respectively.

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