Human responses to positively and negatively valenced stimuli have been examined for years. Many believe that negative stimuli are most salient (recognized faster and more accurately) so that threats can be avoided. Grossmann reviews a variety of evidence leading to the conclusion that human infant displays of fearfulness lead to more care and investment from parents and others.

However, the summary of fear perception and responsiveness through human maturation is incomplete. Infant and toddler responses to emotional faces are not unequivocal. Grossmann's own research shows this (Jessen & Grossmann, Reference Jessen and Grossmann2015), where brief presentations of fear and happy expressions were compared, with event-related brain potentials (ERPs) larger for happy faces than fear in some cases and not others. These inconsistent results match those reported in extensive reviews of the literature on infant expression recognition (Quinn et al., Reference Quinn, Anzures, Izard, Lee, Pascalis, Slater and Tanaka2011; Ruba & Repacholi, Reference Ruba and Repacholi2020; Walker-Andrews, Reference Walker-Andrews1997). The infant expression discrimination and recognition literature shows: (1) Significant variability in the behavioral response to emotional faces before 1 year of age (using measures like categorization and matching; e.g., Kotsoni, de Haan, & Johnson, Reference Kotsoni, de Haan and Johnson2001; Peltola, Leppänen, Palokangas, & Hietanen, Reference Peltola, Leppänen, Palokangas and Hietanen2008; Serrano, Iglesias, & Loeches, Reference Serrano, Iglesias and Loeches1992); (2) a happy face bias in early infancy (e.g., Barrera & Maurer, Reference Barrera and Maurer1981; Farroni, Menon, Rigato, & Johnson, Reference Farroni, Menon, Rigato and Johnson2007; Field, Woodson, Greenberg, & Cohen, Reference Field, Woodson, Greenberg and Cohen1982; Kotsoni et al., Reference Kotsoni, de Haan and Johnson2001); and, (3) a fear (or angry) face bias that emerges at 7 months, but then diminishes by age 11–12 months (e.g., Peltola, Hietanen, Forssman, & Leppänen, Reference Peltola, Hietanen, Forssman and Leppänen2013; Serrano et al., Reference Serrano, Iglesias and Loeches1992; Xie, McCormick, Westerlund, Bowman, & Nelson, Reference Xie, McCormick, Westerlund, Bowman and Nelson2019). Many articles cited here report inconsistent results across multiple experiments, highlighting the uncertainty about infant and toddler facial expression responses. One could easily argue that cooperative care evolved because of a happy face bias by citing different articles on infant and toddler expression recognition. In short, the literature review is incomplete, and does not reflect the complexity of interpreting infant responses to emotional stimuli (notably facial expressions) across all ages.

In addition to the lack of clear “fearfulness” bias in perceiving or discriminating among emotion faces in human infants, there are questions about whether infants under 1 year of age are extracting affect information from faces unless there are multi-modal stimuli (Kotsoni et al., Reference Kotsoni, de Haan and Johnson2001; Quinn et al., Reference Quinn, Anzures, Izard, Lee, Pascalis, Slater and Tanaka2011; Walker-Andrews, Reference Walker-Andrews1997). Conservative interpretations of the results across a myriad of studies suggest high contrast features drive the effects observed (like eye whites or toothy smiles, e.g., Whalen et al., Reference Whalen, Kagan, Cook, Davis, Kim, Polis and Johnstone2004). Just because an infant looks longer at a stimulus it does not mean affect, or the possible consequences of that stimulus, are understood by the child (Quinn et al., Reference Quinn, Anzures, Izard, Lee, Pascalis, Slater and Tanaka2011; Walker-Andrews, Reference Walker-Andrews1997). Research on older children shows significant developmental changes in successfully categorizing facial emotions as generally positive or negative (Gao & Maurer, Reference Gao and Maurer2009, Reference Gao and Maurer2010; Vesker, Bahn, Degé, Kauschke, & Schwarzer, Reference Vesker, Bahn, Degé, Kauschke and Schwarzer2018).

In older children and adults, happy faces are processed faster and more accurately than other emotional facial expressions when artifacts are eliminated and stimuli are equated for salience (Beall & Herbert, Reference Beall and Herbert2008; Calvo & Lundqvist, Reference Calvo and Lundqvist2008; Durand, Gallay, Seigneuric, Robichon, & Baudouin, Reference Durand, Gallay, Seigneuric, Robichon and Baudouin2007; Gao & Maurer, Reference Gao and Maurer2010). The relative salience of negative (usually angry) and positive (happy) faces has been tested using visual search or attentional blink (reviewed by Brosch, Pourtois, & Sander, Reference Brosch, Pourtois and Sander2010; Yiend, Reference Yiend2010); Stroop-type tasks (Beall & Herbert, Reference Beall and Herbert2008; Williams, Mathews, & MacLeod, Reference Williams, Mathews and MacLeod1996; Yiend, Reference Yiend2010); the Simon task (e.g., Altarriba & Basnight-Brown, Reference Altarriba and Basnight-Brown2010); as well as lexical or semantic decision-type tasks (Brosch et al., Reference Brosch, Pourtois and Sander2010; Yiend, Reference Yiend2010); and dot-probe tasks (Sutton & Lutz, Reference Sutton and Lutz2019). Across a myriad of studies one finds evidence for a positivity or negativity bias in the perception of facial expressions across age, although studies on children demonstrate a consistent positive emotion bias (Gao & Maurer, Reference Gao and Maurer2009, Reference Gao and Maurer2010; Kauschke, Bahn, Vesker, & Schwarzer, Reference Kauschke, Bahn, Vesker and Schwarzer2019).

The argument that speeded perception of negative faces may confer an evolutionary advantage has rested on pop-out of emotional faces (the anger superiority effect, Öhman, Lundqvist, & Esteves, Reference Öhman, Lundqvist and Esteves2001). This face in the crowd effect was likely driven by a contrast artifact (Purcell & Stewart, Reference Purcell and Stewart2010; Purcell, Stewart, & Skov, Reference Purcell, Stewart and Skov1996). Savage, Lipp, Craig, Becker, and Horstmann (Reference Savage, Lipp, Craig, Becker and Horstmann2013) demonstrated an anger superiority effect with some faces, and not others. Overall, pop-out of facial expressions does not occur without the presence of low-level stimulus differences, and the anger superiority effect is inconsistent (Calvo & Nummenmaa, Reference Calvo and Nummenmaa2008; Coelho, Cloete, & Wallis, Reference Coelho, Cloete and Wallis2010; Purcell & Stewart, Reference Purcell and Stewart2010; Purcell et al., Reference Purcell, Stewart and Skov1996; Yiend, Reference Yiend2010). A happiness superiority effect obtains when carefully controlled stimuli are used (Craig, Becker, & Lipp, Reference Craig, Becker and Lipp2014).

Based on his review of emotional expression perception by infants Grossmann develops a tenuous line of reasoning that increased attention to fear faces at 7 months can be considered a sign of increased fearfulness among humans, which can then be correlated with later behavioral cooperation in childhood. An unbiased review of the current perspective on infant facial emotion perception at the neural and behavioral levels indicates that any “fearfulness” bias is short-lived, and is bookended by a bias toward perceiving happy faces more quickly and more accurately. Recent attempts to identify individual infants who have heightened fear bias have produced inconsistent results, even when focusing on a narrow age range (5–8-month-olds) using a matching task involving visual disengagement from fearful (compared to other emotional) faces (Kataja et al., Reference Kataja, Leppänen, Kantojärvi, Pelto, Häikiö, Korja and Karlsson2020). While longitudinal studies may clarify whether individual differences in facial emotion expression are predictive of later behavioral effects (e.g., Wagner, Keehn, Tager-Flusberg, & Nelson, Reference Wagner, Keehn, Tager-Flusberg and Nelson2020), the idea that a general fear bias drives cooperation among humans is unsupported. Many researchers have suggested that familiarity with emotional expressions drives the effects, rather than infants understanding fearful expressions to convey fear (e.g., Walker-Andrews, Reference Walker-Andrews1997). The link between an infants' changing preferences for particular facial expressions, their understanding of affect, and how these influence cooperative skills demands direct longitudinal research.