The results of the present study indicate that infants aged from 8 to 22 months exhibit differential mouthing responses to food odours that were classified as pleasant or unpleasant by an adult panel. A first finding was that infants’ responses considered to express avoidance were clearer than responses considered to express attraction. In our conditions, infants could show negative appreciation for odours by manifesting less mouthing responses than the control, and conversely they could show positive appreciation by expressing more mouthing responses than the control. It came out that most of the odours that were selected because they were unpleasant for adults of the same culture, and because they corresponded to foodstuffs known to be avoided by children and infants, elicited reduced mouthing responses (that is, trimethylamine in 12-month-old, dimethyl disulphide in 12- and 22-month-old, and butyric acid in 22-month-old infants). However, the same analyses on the odours chosen because they were pleasant to adults and represented foodstuffs generally liked by children resulted in the absence of strong positive responsiveness at any age (in comparison to control stimuli). While not obviously attractive to the infants, these pleasant odours were however not repulsive, as indicated by the fact they did not elicit decreased mouthing responses, with one notable exception (vanilla) that will be developed below. Thus, in the present experimental conditions, most odours that are pleasant for adults appeared to be treated by 8- to 22-month-old infants as hedonically neutral (that is, not different from the control stimuli). It is worth noting that our study was carried out when the infants were not hungry, at least as reported by their mothers, and deduced from the time of the last feed prior to the test. Thus, their motivation to investigate food-related stimuli may not be maximal, and even more so as these stimuli were presented by the means of bottles. It cannot be excluded that the hedonic responses to the pleasant food odorants might have been exacerbated if infants had been in hunger state, but this is a point of future enquiry. As expected, participants’ hedonic responses indicated avoidance for most of the unpleasant odorants tested here, and this might reflect the dislike ratings for these odours by the adult panel. However, the participants’ hedonic responses were clearly not aligned with those of adults for the pleasant odours. Multiple explanations can be advanced to figure out this asymmetric hedonic response pattern of infants to the present set of stimuli.
First, although previous studies showed that neonates and children older than 3 years of age express an adult-like pattern of olfactory preferences [8, 11], their results must be carefully examined. In Steiner’s work assessing neonates’ facial responses while exposing them to highly concentrated odour stimuli, the most unambiguous negative facial actions were released by the stimuli that were unpleasant to adults. The neonates’ facial responses to the pleasant odour were not as clear-cut, and, accordingly, the corresponding between-observer agreement was medium to low. For example, the infant’s facial responses to the fruity odour (banana) was rated as expressing acceptance, rejection, and indifference in 55, 20 and in 25% of the participants, respectively; similarly, 46% acceptance, 46% indifference and 8% rejection ratings were assigned to infants’ facial reactions elicited by the vanilla odour. Thus, infants and adults did not appear to attribute equivalent hedonic value to odours, and this is clearer on the side of odours considered pleasant to adults. Steiner  himself noted, but without further elaboration, a difference in neonates’ responses to pleasant versus unpleasant odours in terms of hedonic clarity of their facial reactions (‘…the appearance and the course of the reaction to “pleasant odours” was more hesitant or sluggish [than those to “unpleasant” odours]’; p. 274). A later study on neonatal hedonic responses using highly diluted, intensity-matched pleasant and unpleasant odour stimuli supported the notion that neonates do not appraise odour hedonics as adults do, in that they react positively to some odours that adults find aversive, and conversely . Finally, although they demonstrated an overall higher convergence between children and adults, studies on older participants also highlighted age-related discrepant hedonic responses to pleasant odours, while unpleasant odours generated more unanimous responses. For example, in Schmidt and Beauchamp’s study  in 31- to 38-month-old infants, the participants responded differently from adults to odours among both pleasant and unpleasant representatives in the odour series. Thus, the results of the present study, not only corroborate previous studies in younger and older participants in showing different hedonic evaluation of odours by infants and adults, but they highlight that this age-dependent difference is more pronounced for odours that are not aversive to adults. In other words, food odours that are unpleasant to adults - at least those tested in our study - can be predicted with some reliability as also unpleasant for infants, while it is more difficult to predict how infants will perceive food odours that are pleasant to adults. A possible sensory basis of the differential responses induced in infants by the pleasant versus unpleasant odours in the present study will be further developed below.
A second explanation of the asymmetry in hedonic responses to pleasant/unpleasant odours may be related to the design of the present study, which might have accentuated contrasts between the stimuli presented within a same triplet. The within-triplet presentation order of the stimuli was intended to limit the infants’ loss of compliance and attention, so unpleasant stimuli were systematically administered last (first, scentless control; second, pleasant odour; third, unpleasant odour; see Methods section). In this way, we could have created contrast effects (that is, control-pleasant and pleasant-unpleasant), as well as affective carry-over effects from the pleasant odour on the unpleasant odour. Thus, control-pleasant contrasts might have increased the sensory salience of pleasant odours, while pleasant-unpleasant contrasts might have either magnified perception of unpleasant odours due to a quality contrast, or attenuated it due to a carry-over effect of pleasant appraisals onto unpleasant appraisals. As these effects were not systematically manipulated so that all contrasts are represented, any final statement is unwarranted. What can be noted, however, is that the control-pleasant contrasts did not enhance the infants’ attraction as indexed by the mouthing response to the stimulus bottles containing the pleasant odours. Regarding the pleasant-unpleasant odour contrasts, it cannot be decided whether they magnified or attenuated avoidance responses to unpleasant stimuli, but such avoidance responses were high anyway.
It can also be suggested that the consecutive presentation of stimuli can lead to a boredom effect magnifying avoidance responses to unpleasant odours. These stimuli were always presented third and last in the sequence, and are compared to the controls, which were presented first. If a systematic boredom effect had occurred, the scores calculated for the unpleasant odours would have been significantly lower than 0.5. However, the present results did not systematically indicate differences between control and unpleasant stimuli (scores are significantly lower than 0.5 for trimethylamine and dimethyl disulphide at 12 months, and for dimethyl disulphide and butyric acid at 22 months). Thus, the avoidance responses observed towards the unpleasant odours mentioned above are more likely due to the perception of hedonic valence than to a potential boredom effect.
A third explanation of the asymmetry in hedonic responses to pleasant/unpleasant odours may be that the pleasant stimuli were unfamiliar, whereas the unpleasant stimuli were both unfamiliar and conveyed trigeminal potency. Several studies showed indeed that unfamiliar odours are treated as either hedonically neutral  or aversive . In our conditions, the stimuli considered pleasant evoked neither attraction, nor avoidance responses (with the exception of vanilla; see below) in 8-, 12- and 22-month-old infants. Regarding unpleasant stimuli, their unfamiliar quality is obviously confounded with irritant properties as reported by adults (see below, Methods section). Thus, the infants’ avoidance reaction towards unpleasant odours could be explained in part by the trigeminal component of the odours. This hypothesis is backed by adult data on these odours, showing that irritation ratings and pleasantness ratings are negatively correlated (tau = −0.40, P <0.001). However, trigeminal side effects do not explain avoidance responses to all odours. For example, whereas the odours of strawberry and butyric acid did not differ significantly in terms of irritation ratings by adults, strawberry odour did not induce avoidance while butyric acid odour did. Finally, vanillin elicited avoidance behaviour (reduced mouthing at 22 months), despite the fact that this compound is typically regarded devoid of trigeminal properties [24, 25], and was the least irritating in the present odour series. Thus, the negative impact of unpleasant odours in our study cannot be exclusively attributed to confounded trigeminal features.
Although the various explanations offered above may have contributed separately or in combination to the present pattern of findings (that is, an asymmetry in hedonic responses to pleasant/unpleasant odours), our data cannot fully tell them apart. Nonetheless, the main results of a differential hedonic responsiveness of 8- to 22 -month-old infants to pleasant and unpleasant odours are in line with studies conducted on earlier and later ages (see references in the Introduction). Taking the present findings together with earlier published data, it may be generally concluded that infants and children appear to be more reliable in their negative responses than in their positive responses to odours. For example, while the facial actions expressing disgust do accurately differentiate butyric acid from vanilla odours, those expressing smiles are not discriminant [7, 26]. In sum, during early development, odour-related hedonic processes may be better integrated on the negative pole than on the positive pole of the hedonic space .
The finding on vanilla odour was unexpected: despite vanilla being rated as highly pleasant by adults, it induced avoidance in 22-month-old infants. Vanilla odour is assumed to be one of the most familiar odorants in the present stimulus series as it is a regular aroma component of formula milk and infant foods. Two processes can be proposed to explain infants’ avoidance of this particular odorant in the present conditions. First, it is known from previous infant studies that frequent and/or recent exposures to a specific flavour lead to a boredom effect, thus altering an infant’s responsiveness to it [28, 29]. For example, an increase in acceptance for carrot-flavoured cereals after exposure to carrot flavour through mother’s milk was noted when the delay between last exposure and acceptance testing was from 4 to 6 months , but not when it was only 3 days . Second, an alliesthesia effect may have operated, infants responding rather negatively to odours and flavours that dominated in their food. Satiation-related factors were indeed shown to reduce liking of food odour in neonates  and older children , and there is no reason why such motivational factors should not also affect infants of intermediate ages although age differences in alliesthesia effects were shown in later development . Finally, and in line with the previous effect, it cannot be excluded that the test-bottle used in the present study could be reminiscent of the bottle from which the infants drank beverages. Since most formula milk for older infants are vanilla-flavoured, infants may have expected a reward when presented a vanilla-scented bottle. This expectation not being satisfied in the test, infants may have exhibited less mouthing.
This study assessed the development of hedonic responses to odours at three time points in the first 2 years of life. When considering the 8 odours separately, no significant difference in mouthing score was noted at 8 months, whereas two significant differences were observed at 12 months (for trimethylamine and dimethyl disulphide). Finally, three significant differences were observed at 22 months (dimethyl disulphide, butyric acid and vanillin). All but one of these differential odour-based mouthing responses concerned unpleasant odours. One could argue that infants might exhibit increasingly sharper avoidance behaviour when they grow older. The progressive emergence of neophobia  could explain this behavioural change.
As regards the correlations between mouthing scores for the same odorant at two different ages, some significant correlations were noted only for unpleasant odours. Moreover, if we look at the individual correlations calculated between ages, only a few were significant (about 6% of all correlations tested). Thus, very few infants display the same pattern of mouthing behaviour towards the odours between two different age points. These results suggest both inter- and intra-individual differences in the development of the hedonic perception of the odours. Given that the organization of the human olfactory epithelium may reflect key dimensions of olfactory perception (odorant pleasantness) , one may think that this organization is stable and inflexible. Nevertheless, this mapping of odour perception is malleable by context and experience . Thus, either positive or negative context of previous exposures can contribute to the uniqueness of each individual’s development of the hedonic appraisal of odours or flavours [34, 35]. Alternatively, the emergence of food neophobia could also explain individual variability in the development of hedonic perception of odours. This phenomenon could happen more or less early depending on infants, and its strength could differ as a function of an infant’s temperament [17, 36]. Individual variability from one age to the other suggests plasticity of olfactory responses across time, which is particularly important in the formation of positive responsiveness to odours. This assumption is backed by a follow up study which indicated a significant increase in liking of food odours between the ages of 3 and 5 years . By contrast, the present results indicate that infants’ responses to the unpleasant odours are partially stable across ages. Moreover, the follow-up study mentioned above on 3- to 5-year-olds showed that there is no significant change of dislike for odours classified as toxic . It seems that odours related to potential toxic or harmful foods are considered as unpleasant - and are actually avoided in laboratory studies - early in life, and remain unpleasant and avoided when infants grow up (at least when presented only as chemosensory stimuli). This response might constitute an olfactory alarm system protecting against potentially toxic food. Finally, it has been shown that 6- to 12-year-old children from different ethnic backgrounds (French Canadians, Sudanese Indonesian, and Syrian) agreed on the odours they judged as being unpleasant but not on those judged as being pleasant , highlighting the relative consensus of children’s responses towards unpleasant odours relative to pleasant ones.