Paola Bressan

Paola Bressan has written 10 posts for Paola Bressan

Humans as superorganisms: how microbes, viruses, imprinted genes, and other selfish entities shape our behavior

Peter Kramer, Paola Bressan


Psychologists and psychiatrists tend to be little aware that (a) microbes in our brains and guts are capable of altering our behavior; (b) viral DNA that was incorporated into our DNA millions of years ago is implicated in mental disorders; (c) many of us carry the cells of another human in our brains; and (d) under the regulation of viruslike elements, the paternally inherited and maternally inherited copies of some genes compete for domination in the offspring, on whom they have opposite physical and behavioral effects.

This article provides a broad overview, aimed at a wide readership, of the consequences of our coexistence with these selfish entities. The overarching message is that we are not unitary individuals but superorganisms, built out of both human and nonhuman elements; it is their interaction that determines who we are.

Kramer, P., & Bressan, P. (2015). Humans as superorganisms: how microbes, viruses, imprinted genes and other selfish entities shape our behavior. Perspectives on Psychological Science, 10, 464-481. doi: 10.1177/1745691615583131


Modes of invasion of humans by selfish entities.

Read full paper


• read coverage in BBC Future (“Is another human living inside you?”, 18 September 2015). This story is part of BBC Future’s “Best of 2015” list, their greatest hits of the year.

• read coverage in New York Magazine (“Adventures in the Science of the Superorganism”, 5 October 2015)

• read coverage in Psychology Today (“How Microbes, Viruses, Imprinted Genes, etc. Shape Behaviour”, 9 January 2016)



Time estimation predicts mathematical intelligence

Peter Kramer, Paola Bressan, Massimo Grassi

Performing mental subtractions affects time (duration) estimates, and making time estimates disrupts mental subtractions. This interaction has been attributed to the concurrent involvement of time estimation and arithmetic with general intelligence and working memory. Given the extant evidence of a relationship between time and number, here we test the stronger hypothesis that time estimation correlates specifically with mathematical intelligence, and not with general intelligence or working-memory capacity.

Participants performed a (prospective) time estimation experiment, completed several subtests of the WAIS intelligence test, and self-rated their mathematical skill. For five different durations, we found that time estimation correlated with both arithmetic ability and self-rated mathematical skill. Controlling for non-mathematical intelligence (including working memory capacity) did not change the results. Conversely, correlations between time estimation and non-mathematical intelligence either were nonsignificant, or disappeared after controlling for mathematical

We conclude that time estimation specifically predicts mathematical intelligence. On the basis of the relevant literature, we furthermore conclude that the relationship between time estimation and mathematical intelligence is likely due to a common reliance on spatial ability.

Kramer, P., Bressan, P., & Grassi, M. (2011). Time estimation predicts mathematical intelligence. PLoS ONE, 6(12): e28621.

See original paper on the PLoS ONE website (the pdf can be downloaded for free)

Why is our research important?
Estimating beep durations requires no calculation. Yet, we found that the best duration estimators tend to have the highest mathematical intelligence. What most likely underlies both time estimation and mathematical ability is a tendency to mentally represent quantities in a spatial way. Encouraging this tendency might help improve mathematical intelligence, satisfying one of modern society’s greatest needs.


• read coverage in Inkfish (“Why good time estimators are better at math”, 8 December 2011)

Men do not have a stronger preference than women for self-resemblant child faces

Paola Bressan, Marco Bertamini, Alessandra Nalli, Arianna Zanutto

Are men more likely than women to take into account a child’s facial resemblance to themselves when making hypothetical parental investment choices? The benefits of self-resemblance in decreasing relatedness uncertainty are larger in men than in women for direct descendants. However, they are identical in men and women for collateral relatives, such as siblings, cousins, nephews, and nieces; these individuals can also be the recipients of parental-like altruism, which comes primarily from women. Published data are contradictory. In the present study, 14 men and 14 women were shown child faces and asked to judge their attractiveness, adoptability, and familiarity. The faces had been digitally manipulated to resemble (at three different resemblance levels, two of which were under recognition threshold) either the experimental participant, an acquaintance, or strangers. We found a significant preference for self-resemblant children in women, but not in men. This was not an artefact of women being better at detecting self-resemblance, given that at the highest resemblance level more men than women recognized themselves. Overall, face preference increased with face familiarity; for self-resemblant faces, this correlation was not mediated by conscious self-recognition. We discuss how the fast-response, multiple-question procedure used in previous experiments may have led to reports of a much larger self-resemblance preference in men than in women.

Bressan, P., Bertamini, M., Nalli, A., & Zanutto, A. (2009). Men do not have a stronger preference than women for self-resemblant child faces. Archives of Sexual Behavior, 38, 657-664.

child morphs

These faces are morphs, containing 25%, 40%, and 50% of a female participant's traits.

Read full paper

Human kin recognition is self- rather than family-referential

Paola Bressan, Guendalina Zucchi

Inclusive fitness theory predicts that organisms will tend to help close kin more than less related individuals. In a variety of birds and mammals, relatives are recognized by comparing their phenotype to an internal representation or template, which might be learned through either repeated exposure to family members or self-inspection. Mirrors are ubiquitous now, but were absent during our evolutionary history; hence it is hard to predict, and empirically unknown, whether human kin recognition is family- or self-referential. Here we put this issue to the strongest possible test by comparing nepotistic behaviour towards self- versus co-twin-resemblant individuals. Seventy monozygotic and dizygotic twins were shown same-sex faces, covertly manipulated to resemble either themselves or their co-twin, and indicated which individual they would prefer in two prosocial contexts. Self-resemblant faces were significantly preferred to twin-resemblant faces, showing that visual information about the self supersedes that about close family members in the kin-recognition template. Because, under conditions of paternal uncertainty, a reliable family-referent template could be based only on one’s mother and maternal relatives, a unique advantage of self-referent phenotype matching is the possibility of (consciously or unconsciously) identifying one’s father and paternal relatives as kin.

Bressan, P., & Zucchi, G. (2009). Human kin recognition is self- rather than family-referential. Biology Letters, 5, 336-338.

Two identical twins morphed with the female model

Because monozygotic twins are identical genetically, but not developmentally, morph pairs were very similar but never impossible to tell apart.

Read full paper


• read coverage in the Telegraph (“We are friendlier to people who resemble us, scientists find”, 4 March 2009)
• read coverage in Focus (“Je ähnlicher, desto hilfsbereiter”, 4 March 2009)
• read coverage in Stern (“Gleich und gleich hilft sich gern”, 4 March 2009)
• read coverage in Wissenschaft (“Das eigene Gesicht als unbewusster Standard”, 4 March 2009)

Biologically costly altruism depends on emotional closeness among step but not half or full siblings

Paola Bressan, Stephen M. Colarelli, Mary Beth Cavalieri

We studied altruistic behaviors of varying biological cost (high, medium, and low) among siblings of varying genetic relatedness (full, half, and step). In agreement with inclusive fitness theory, the relative importance of either reliable (such as co-residence) or heuristic (such as emotional closeness) kinship cues depended crucially on the costs of help. When help did not endanger the altruist’s life, thus making reciprocation possible, emotional closeness was the strongest predictor of altruism; perceived physical and psychological similarity to the sibling amplified altruistic behavior via their association with emotional closeness. When help endangered the altruist’s life, thus making reciprocation unlikely, the strongest predictor of altruism was the ancestrally valid kinship cue of co-residence duration. Emotional closeness predicted costly altruism only for step siblings; its effects were non-significant when siblings were genetically related. Our findings support the idea that emotional closeness promotes costly altruistic behavior by serving as a surrogate kinship cue when more reliable cues are missing.

Bressan, P., Colarelli, S. M., & Cavalieri, M. B. (2009). Biologically costly altruism depends on emotional closeness among step but not half or full siblings. Evolutionary Psychology, 7, 118-132.

Gating of remote effects on lightness

Paola Bressan, Peter Kramer

In various versions of the dungeon illusion (P. Bressan, 2001), we show that grouping between targets and contextual disks determines whether remote luminances affect target lightness or not. In the dungeon illusion, target disks surrounded by contextual disks contrast with them rather than with the immediate background. We formally establish the existence of this illusion and show that it reverses when the luminance of the targets is either lower (double decrement) or higher (double increment) than the luminances of both the background and the contextual disks rather than in between them. On the basis of the double-anchoring theory of lightness (P. Bressan, 2006a), we predict and show that grouping gates the effects of remote luminances in such a way that they go in opposite directions in the double-decrement and double-increment inverted-dungeon illusions. Our results support the double-anchoring theory and demonstrate that luminances that are far away from the targets are irrelevant in some conditions but critical in others.

Bressan, P., & Kramer, P. (2008). Gating of remote effects on lightness. Journal of Vision, 8(2):16, 1-8.

See original paper on the Journal of Vision website (the pdf can be downloaded for free)

color dungeon

The central eight disks in each panel look either bluish or greenish, but they are all actually grey.


The best men are (not always) already taken: Female preference for single versus attached males depends on conception risk

Paola Bressan, Debora Stranieri

Because men of higher genetic quality tend to be poorer partners and parents than men of lower genetic quality, women may profit from securing a stable investment from the latter, while obtaining good genes via extrapair mating with the former. Only if conception occurs, however, do the evolutionary benefits of such a strategy overcome its costs. Accordingly, we predicted that (a) partnered women should prefer attached men, because such men are more likely than single men to have pair-bonding qualities, and hence to be good replacement partners, and (b) this inclination should reverse when fertility rises, because attached men are less available for impromptu sex than single men. In this study, 208 women rated the attractiveness of men described as single or attached. As predicted, partnered women favored attached men at the low-fertility phases of the menstrual cycle, but preferred single men (if masculine, i.e., advertising good genetic quality) when conception risk was high.

Bressan, P., & Stranieri, D. (2008). The best men are (not always) already taken: Female preference for single versus attached males depends on conception risk. Psychological Science, 19, 145-151.


• read coverage in Nature (“Fertile wives find single men sexy”, 26 February 2008)

Visual attentional capture predicts belief in a meaningful world

Paola Bressan, Peter Kramer, Mara Germani

Here we show that the automatic, involuntary process of attentional capture is predictive of beliefs that are typically considered as much more complex and higher-level. Whereas some beliefs are well supported by evidence, others, such as the belief that coincidences occur for a reason, are not. We argue that the tendency to assign meaning to coincidences is a byproduct of an adaptive system that creates and maintains cognitive schemata, and automatically directs attention to violations of a currently active schema. Earlier studies have shown that, within subjects, attentional capture increases with schema strength. Yet, between-subjects effects could exist too: whereas each of us has schemata of various strengths, most likely different individuals are differently inclined to maintain strong or weak ones. Since schemata can be interpreted as beliefs, we predict more attentional capture for subjects with stronger beliefs than for subjects with weaker ones. We measured visual attentional capture in a reaction time experiment, and correlated it with scores on questionnaires about religious and other beliefs and about meaningfulness and surprisingness of coincidences. We found that visual attentional capture predicts a belief in meaningfulness of coincidences, and that this belief mediates a relationship between visual attentional capture and religiosity. Remarkably, strong believers were more disturbed by schema violations than weak believers, and yet appeared less aware of the disrupting events. We conclude that (a) religious people have a stronger belief in meaningfulness of coincidences, indicative of a more general tendency to maintain strong schemata, and that (b) this belief leads them to suppress, ignore, or forget information that has demonstrably captured their attention, but happens to be inconsistent with their schemata.

Bressan, P., Kramer, P., & Germani, M. (2008). Visual attentional capture predicts belief in a meaningful world. Cortex, 44, 1299-1306.

Dungeons, gratings, and black rooms: a defense of double-anchoring theory and a reply to Howe et al. (2007)

Paola Bressan

The double-anchoring theory of lightness (Bressan, 2006) assumes that any given region belongs to a set of frameworks, created by Gestalt grouping principles, and receives a provisional lightness within each of them; the region’s final lightness is a weighted average of all these values. In their critique, Howe, Sagreiya, Curtis, Zheng, and Livingstone (2007) (a) show that the target’s lightness in the dungeon illusion (Bressan, 2001) and in White’s effect is not primarily determined by the region with which the target is perceived to group, and (b) claim that this is a challenge to the theory. I argue that Howe et al. misinterpret grouping for lightness by equating it with grouping for object formation, and by ignoring that lightness is determined by frameworks’ weights and not by what appears to group with what. I show that Howe et al.’s empirical findings, together with those on grating induction and all-black rooms that they cite as problematic, actually corroborate rather than falsify the theory.

Bressan, P. (2007). Dungeons, gratings, and black rooms: a defense of double-anchoring theory and a reply to Howe et al. (2007). Psychological Review, 114, 1111-1115.

The place of white in a world of grays: a double-anchoring theory of lightness perception

Paola Bressan

The specific gray shades in a visual scene can be derived from relative luminance values only when an anchoring rule is followed. The double-anchoring theory I propose in this paper, as a development of the anchoring theory of Gilchrist et al. (1999), assumes that any given region (a) belongs to one or more frameworks, created by Gestalt grouping principles, and (b) is independently anchored, within each framework, to both the highest luminance and the surround luminance. The region’s final lightness is a weighted average of the values computed, relative to both anchors, in all frameworks. The new model not only accounts for all lightness illusions that are qualitatively explained by the anchoring theory, but also for a number of additional effects; and it does so quantitatively, with the support of mathematical simulations.

Bressan, P. (2006). The place of white in a world of grays: a double-anchoring theory of lightness perception. Psychological Review, 113, 526-553.