Exclusion criteria for the study involved the following characteristics: - only children – as they cannot be indicative of how siblings judge kinship cues; - participants who had any “non-full” siblings (including half-, step-, and adopted siblings) – because observation of the perinatal mother-child relationship is unclear; - twins who did not have other full (younger or older) siblings – as birthorder in twins is not related to observation of perinatal mother-baby relationship - participants having more than 2 full siblings – following Kaminski et al. (2010)’s paper Binary relatedness judgments will be analysed using binomial mixed regression in R. The formula is: judgment ~ rel.e * birthorder.e + (1 + rel.e || rater) + (1 + birthorder.e || stim_id). The dependent variable is the relatedness *judgment* (0 = unrelated, 1 = related), the independent variables are relatedness (*rel.e* : -0.5 = unrelated, 0.5 = related) and birth order (*birthorder.e* : -0.5 = firstborn, 0.5 = laterborn). The rater and stimulus ID are included as random effects and slopes are specified maximally. The analysis script has been attached as a R markdown file. The final R analysis script is attached (all non pre-registered exploratory analyses are pointed out clearly), as well as the Power calculation. With 100 participants (50 firstborn/50 laterborn), we have 93% power to detect an interaction between birth order and relatedness with estimate ≅ 0.27 (odds ratio ≅ 1.3) at 5% alpha. As data collection has finished we have now uploaded the data for anyone to run the Analysis script.