Central to the fearful ape hypothesis is the notion that infants expressing heightened fearfulness traits received increased care from mothers and others in our evolutionary history. Crying is conceptualized by Grossmann as a vocal expression of infant fear, and more frequent crying is argued to “elicit more caring behavior.” We agree with the contention that infant crying typically elicits adaptive caregiving and is powerful in securing attention over the protracted period of human infant dependency. However, we would add the caveat that, in modern times, crying is also the most commonly cited trigger for physical abuse in the first months of life. Even if we believe that fearful crying has been broadly adaptive in human evolution, we should not overlook the role of infant crying in maladaptive responses in modern parents. Although Grossmann does discuss how fearful infants may be differentially susceptible to environmental influences, there is no discussion of how adults may be differentially susceptible to infant crying. For many modern parents, crying is commonly perceived as excessive. Prolonged and extended crying is difficult to cope with and is one of the most common reasons for help-seeking from healthcare professionals (St James-Roberts, Hurry, & Bowyer, Reference St James-Roberts, Hurry and Bowyer1993). Crying has been identified as a major risk factor for infant shaking or inappropriate physical actions. In a landmark study of parents in the Netherlands, nearly 6% reported having smothered, slapped, or shaken their infants to stop their crying (Reijneveld, van der Wal, Brugman, Sing, & Verloove-Vanhorick, Reference Reijneveld, van der Wal, Brugman, Sing and Verloove-Vanhorick2004). Actual rates of shaking may be even higher because of parental underreporting (Reijneveld, Reference Reijneveld2007).

Although physical abuse may be at the extreme end of negative outcomes, other non-optimal associations between crying and caregiving have been reported. When caregivers perceive crying as excessive, it can lead to early termination of breastfeeding (e.g., Howard, Lanphear, Lanphear, Eberly, & Lawrence, Reference Howard, Lanphear, Lanphear, Eberly and Lawrence2006) and it can predict the subsequent onset of maternal depression (Petzoldt, Reference Petzoldt2018). Mothers who report that their infants cry a lot also tend to score high on parental stress indices and report less positive reinforcement from their infants (Beebe, Casey, & Pinto-Martin, Reference Beebe, Casey and Pinto-Martin1993).

Why might crying elicit approach and caregiving behaviour in most adults, but precipitate abuse or maladaptive responses in some? The impact of infant crying, or vocal fear expression, may depend on parents' own health, vulnerabilities, prior family history, and current family circumstances. For example, adults at risk for harsh parenting have been shown to be less likely to habituate in response to repeated cry sounds compared to other adults (Out, Bakermans-Kranenburg, van Pelt, & Van Ijzendoorn, Reference Out, Bakermans-Kranenburg, van Pelt and Van Ijzendoorn2012). Mothers with mood disorders show blunted reactivity to infant crying, as measured by eye-tracking indices and galvanic skin response, compared to mothers with no mood disorder (Bjertrup et al., Reference Bjertrup, Moszkowicz, Egmose, Kjærbye-Thygesen, Nielsen, Parsons and Miskowiak2021). Limited social support for the parent is also suggested to increase the negative effects of crying on the parent and infant (St James-Roberts et al., Reference St James-Roberts, Garratt, Powell, Bamber, Long, Brown and Jaicim2019).

A number of the characteristic properties of infant crying have the potential to increase maladaptive responding, together or in isolation. First, crying, as with other forms of fear expression, is an ambiguous signal, and many cry bouts begin and end for no clear reason (Barr, Reference Barr2014). Second, the acoustic properties of crying mean that it is typically perceived as aversive, with its high pitch, unpredictability (Young et al., Reference Young, Parsons, Stein, Vuust, Craske and Kringelbach2019), and “roughness.” Crying can be especially aversive, or frustrating to caregivers when it is prolonged and difficult to soothe (Fujiwara, Barr, Brant, & Barr, Reference Fujiwara, Barr, Brant and Barr2011). If we consider crying not just as a one-off alarm signal, the complexity of the cry–caregiving association becomes clear.

Indeed, crying is a communicative signal that parents must repeatedly respond to over a lengthy developmental period. Meta-analytic evidence from parent reports of crying across countries shows that parents deal with substantial durations of crying over the first 12 months post-birth. Cry duration does not drop off as markedly from its initial peak at 6–8 weeks as earlier described in the classic “cry curve,” whereby crying duration shows a clear and steady decline (Vermillet, Tølbøll, Litsis Mizan, Skewes, & Parsons, Reference Vermillet, Tølbøll, Litsis Mizan, C. Skewes and Parsons2022). Although data on what constitutes typical or normal cry behaviour is still sparse, crying for long periods beyond 6 months of age has been shown to predict later childhood regulatory difficulties (von Kries, Kalies, & Papoušek, Reference von Kries, Kalies and Papoušek2006). What is appropriate fear expression at one age may signal developmental difficulties, problems in parent–infant interactions, or physical health issues, at a later age.

Whether high infant fearfulness, and crying as we have focused on here, elicits enhanced caregiving, depends on the context in which it occurs, and in particular, on the capacities of nearby adults. Taking our broader evolutionary context, as the fearful ape hypothesis does, fearful crying is understood as a stimulus for cooperative caregiving by a familial group. However, a mother in a modern single-family home, deprived of sleep, with limited social support might have different capacities to respond to a high-fear infant than a mother living communally with none of these risk factors. While overarchingly, fear expression and crying might function to promote cooperative group caregiving by adults, if perceived as excessive by the individual listener, it can precede, in the most unfortunate cases, physical abuse.