*4.4.3 ERP procedures*

As all studies were interested in behavioral tasks tapping social or emotional processing (**Table 1**), there were no inquiries about ERP components for other psychological functions such as object perception, attention, memory, language, executive functioning, and motor control. This is a major gap to fill, as behavioral and fMRI studies have shown that OXTR polymorphisms may partly explain the differences in the development of these cognitive processes [6, 70].

The experimental paradigms used in the studies hinder comparison between ERPs components such as P3, Nc, LPP, slow wave potentials, and error-related negativity among OXTR genotypes. In relation to EEG acquisition and preprocessing, we detected wide variation in the number of electrodes, referencing, filtering, sampling rate, artifact detection, elimination, and correction, which makes it hard to compare ERPs results [71]. In general, the reports lacked substantial explanations about the selection of ERP measurements, latency ranges, electrode reduction methods, and statistical procedures. Most researchers used visual inspection of the grand average

to select latency ranges, employed amplitude, and latency peaks and chose electrodes autonomously to calculate each component, being all these procedures discouraged by ERP guidelines [27, 72].

#### *4.4.4 Publication bias*

Our objective was to carry out a systematic review but not a meta-analysis because of the heterogeneity in ERP procedures, psychological paradigms, statistics for hypothesis testing, and number of subgroups compared. All these make very difficult to run quantitative analyses for publication bias, sensitivity, and subgroups. However, in the exploratory analysis we did not find publication bias solid evidence; three of the eight studies report negative results with nonsignificant differences between OXTR genotypes. Moreover, of the 23 components analyzed (**Figure 3**), seven [30%] showed significant differences between genotypes, and in the 70% of comparison, the differences were nonsignificant. However, small sample sizes (**Table 1**), small effect sizes, and high p-value (<0.05) in two studies with positive results could mean a possible bias in these publications [22, 23]. In the future, editors could demand higher effect sizes and narrower confidence intervals in the reports of quantitative results, as suggested in the APA guidelines, which may facilitate the comparison of studies.

In short, future studies will benefit from the use of larger samples, more heterogenic aged populations in the samples, and the recruitment of participants from different geographical places around the world. Additionally, it will be helpful for studies to include sex or gender as a control variable, especially when the samples include adolescents. Also, it may be advantageous for the field to demand that studies use reliable, conventional, and standardized protocols to acquire and process EEG signals. Accumulating evidence from future studies will help to establish, refute, and clarify how EPR components function as a window into the oxytocinergic processing of human psychological functions.
