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One- to four-year-olds connect diverse positive emotional vocalizations to their probable causes

Significance We find that very young children make fine-grained distinctions among positive emotional expressions and connect diverse emotional vocalizations to their probable eliciting causes. Moreover, when infants see emotional reactions that are improbable, given observed causes, they actively search for hidden causes. The results suggest that early emotion understanding is not limited to discriminating a few basic emotions or contrasts across valence; rather, young children’s understanding of others’ emotional reactions is nuanced and causal. The findings have implications for research on the neural and cognitive bases of emotion reasoning, as well as investigations of early social relationships.

had an effect on accuracy at all rather than in its specific relationship (e.g., linear, quadratic, or cubic) to accuracy. We used likelihood ratio tests to see whether any interaction term or the autocorrelation structure of order 1 would significantly improve the model. The final best-fit model included the interaction between Time and Age, and the autocorrelation structure of order 1. See Tables S3 and S4.
The post-hoc analysis of age used the same best-fit model except that instead of taking Age as a continuous variable, we did a median split on age and looked at the effects of Time and Category on each of the two age groups. See Table S5. We used the same model to analyze the data replicating the older age group. See Table S6. (In this replication study, we also initially pre-registered an analysis to investigate the possibility of a quadratic curve in 18-23-month-olds' preferential looking over the 7-second interval; however, we elected not to pursue this line of analysis and instead treated Time as a categorical variable.)

Experiment 3
We analyzed the effects of Time, Category, and Age on participants' performance. We calculated participants' accuracy on each of the seven seconds on each trial, averaged across the two 7-second intervals (see Fig. 2A). Then we averaged across trials involving each category of the eliciting cause, either as a target or a distractor (note that, as above, this controls for baseline preferences). As in the previous experiments, we used likelihood ratio tests to select the final best-fit model. It did not include any interaction term but included the autocorrelation structure of order 1. See Tables S7 and S8.

Experiment 5
We were interested in the effects of Congruency, Category, and Age on participants' performance. By design, Congruency (a categorical variable), Category (a categorical variable), and Age (a continuous variable) were fixed factors, and Subject was a random factor. The likelihood ratio tests suggested that no interaction term should be added to the final best-fit model. See Tables S9-S10. 2. Pre-registration for Experiment 5 In addition to the four analyses reported in the paper, we also mistakenly pre-registered an incorrect analysis: a mixed-effects model looking at the effects of Congruency, Category, and Age on infants' proportional searching in the Congruent and Incongruent conditions. However, because each participant's proportional searching in the two conditions always summed to 1, this analysis cannot estimate the main effects of Category and Age and we did not run it.

Pilot study for Experiment 4
We conducted a pilot study including 16 12-17-month-olds (M: 15.3 months, range: 12.0-17.6). There was a trend for participants to search longer in the Incongruent than the Congruent

Exploratory analyses of Experiment 4
Exploratory analyses collapsing the pilot sample (N=16) and the main sample (N=36) suggested that the effect between conditions was present in 15-17-month-olds but absent in 12-14-month-olds. The older group (n=24) searched significantly longer in the Incongruent than the
6. Procedure of the practice/training trials 6.1 Experiment 1 For the practice trial, the experimenter displayed two warm-up pictures (i.e., a beautiful beach and a dying flower) on the screen and said: "In this game I will show you two pictures.
Sally will look at one of them and make a sound. We will need to guess which picture Sally is looking at. OK? I'll play the game first!" The experimenter played the positive sound, pointed to the picture of the beach, and said: "When Sally makes this sound, I think she is looking at the picture of a beautiful beach." Then she played the sad sound, pointed to the dying flower, and said: "When Sally makes this sound, I think she is looking at this dying flower." Then the experimenter said: "Now it's your turn to play the game!" and started the test trials.

Experiments 4 and 5
During the training phase, the experimenter took out a box and said: "Now let's play a game! This is my box.
Let me see what's inside." The experimenter looked into the box through the top hole, saying: "I see something!" She reached through the felt opening and retrieved a ball. She looked at the child and said: "Look! It's a ball!" She squeaked the ball three times and put it in the tray. Then she said: "Let me see what else is inside!" She searched the box for approximately three seconds and then opened her hand to show there was nothing on it, saying: "Nothing else!" Then the experimenter put the ball back in the box and affixed the box to the table with the felt opening facing the child. The experimenter told the child: "Now it's your turn to play! Take the ball out and put it in the tray!" After the children retrieved the ball, they were encouraged to place it in the tray. Once they did, the experimenter took the ball away. A similar procedure was repeated with the other training box. When the experimenter searched the box the first time, she retrieved a red ball. When she searched the box again, she retrieved a toy car (Experiment 4) or a toy banana (Experiment 5). Then she searched the box again and found nothing.

Experiments 2 and 3
In both Experiments 2 and 3, a coder, blind to both the eliciting causes and the vocal expressions, coded the children's looking offline from videotape during each 7-second interval (the 4 seconds when the vocalization was played and the 3 second pause that followed it). A second coder coded 33% of the clips. Inter-coder agreement was 90% for Experiment 2 and 96% for Experiment 3. Accuracy was calculated as children's looking time to the image corresponding to the vocalization over their total looking time to both images during the 7seoncd intervals.

Experiments 4 and 5
Four behaviors were coded as "searching": reaching into the box through the front opening, trying to reach into the box through the top hole, trying to look through the front opening, and trying to look through the top hole. All other behaviors (e.g., pushing the box, playing with the felt, and resting a hand on the top hole) were coded as "not searching". The coding criteria were the same for Experiments 4 and 5. Coders were trained together and then coded the video clips separately. All coders were blind to the experimental conditions. In Experiment 4, a primary coder coded all video clips, and a second coder coded 90% of the video clips. Inter-coder agreement was 92%. In Experiment 5, a primary coder coded all video clips and a second coder coded 60% of the video clips. Inter-coder agreement was 95%.    Singularity in backsolve at level 0, block 1