Babies aged 4 to 6 months link saliva sharing to close bonds in UCSB-led looking-time tests

The first test left two easy explanations open. The saliva sharer and the ball-giver were different people, and only one of them offered food ^[3]. A baby drawn to food, or to one particular face, could produce a looking difference without any idea of who is close to whom. The finger test removed both problems: one person, no food ^[4]. According to the report, it was included to check whether infants tracked the saliva exchange itself or the food ^[6].

The two tests also pointed the distress in opposite directions. In the first, the puppet was upset, and researchers recorded which person the infant looked at longer ^[3]. In the second, the person "cried", and they watched which puppet the infant expected to respond ^[4]. Both times the longer look went to the partner who had shared saliva ^[5]. I think the second test is the stronger evidence. It holds the person constant and drops the food, and the result came out the same way ^[4][5].

Woo said the finger action "is not as common in children's everyday lives" ^[10]. The babies still treated it as a sign of closeness ^[5]. "It's fascinating that they seem to have this abstract knowledge about relationships that applies across different actions," he said ^[11]. The new tests place the inference two to four months earlier than the group's previous estimate ^[15].

The starting point was adult behaviour. "We were inspired by studies that found that across cultures, people in close relationships are often more comfortable sharing saliva with each other," Woo said ^[9]. His case for why infants would track this is practical. "Because it's so important for children to work with others and collaborate with others, they need to map out these social networks," he said ^[13].

The phys.org report does not give the number of infants tested, the size of the looking-time differences, or any measure of how much saliva sharing each baby had seen at home ^[14]. Without those figures, the size of the effect cannot be judged from the report.

Woo places the work in a long argument about what infants know. "Some scholars argue that babies are blank slates that know nothing," he said. "This work is aiming to get at just how babies are starting to make sense of the social world, of social relationships, and what the role of experience is." ^[12] Compared with older children, babies of 4 to 6 months have had less opportunity to observe saliva sharing, and they are less likely to look at faces ^[8].