It is well known that different person have specific psychological meaning to us. We usually feel more positive to intimates (kin) than relatively non-intimate ones (non-kin). However, until recently, the underlying mechanism about kinship remains poorly understood. Thus, we further investigated whether the degree of kinship between a perceiver and a target person leads to the perceiver’s specific electrophysiological response. Event-related brain potentials (ERPs) of 22 participants (age: 20.8 ± 2.13) were observed when changed the degree of kinship (father, uncle, acquaintance) in morally laden scenarios. Our results demonstrated that the amplitudes of neural response varied among kins and acquaintance. Specifically, fronto-central positive activity at 180 - 230 ms (P2) and central-parietal late positive activity at 350 - 500 ms (LPC) were of larger mean amplitude in response to father than to uncle and acquaintance, which are indicative of intense information processing and sensitivity to a lineal relative in moral cognitive context. Those findings showed direct evidences of consanguineous bias in moral-related contexts, which will provide valuable reference for intervention of tensioned relationship and other related disorders.
Kin selection theory (KST) or inclusive fitness theory refers to more positive attitude or altruistic behavior to kins relative to others, which is naturally seen as a product of biological evolutionary [
Taken together, though previous studies focused on kin-serving bias, the category standard is vague, and merely dividing kinship by kin, acquaintance and stranger using anonymous agents [
Based on previous ERPs studies [
As paid volunteers, 22 undergraduate students (10 women, 12 men) aged 19 - 23 years (mean age: 20.8 ± 2.13), in Guangxi Teachers Education University participated in the experiment. All participants were right-handed, had normal or corrected-to-normal vision, and had no current or past neurological or psychiatric illness. All gave written informed consent before participating.
We chose eight story segments (187 - 225 Chinese words in length, refer to [
Experimental materials were presented on a 17-in. computer monitor screen, including the morally laden scenarios, the three types of stimuli (the names of every subject’s father, uncle and an acquaintance), and several interpolated displays (e.g., a fixation cross). Target names were displayed in random order in the center of the screen. The size of each name was approximately 1.72 cm (horizontal) × 0.92 cm (vertical). It should be noted that, for Chinese names, the names of father and uncle, sometimes even an acquaintance may look very similar (e.g., Cheng Jianguo, Cheng Jianzu, Cheng Ziqiang, and may elicit very similar visual response).
Subjects were seated in a semi-dark room facing the computer monitor placed 60 cm from their eyes, and the horizontal and vertical visual angles subtended by the stimuli were below 5˚. We instructed participants to keep their eyes fixated on the monitor throughout experimental trials. The total experiment included practice phase (a single scenario and 10 trials, data not included in analysis) and test phase (8 scenarios or blocks and 240 trials, 30 trials per block in which 10 trials for each condition, randomized). In each block of the test phase, a written scenario was first presented on the screen. After the subject finished reading the scenario, he or she pressed a key, and a plus sign appeared in the center of screen for 300 ms, followed by a blank screen that persisted for 500 - 800 ms. Then the name of the subject’s father, uncle or acquaintance was presented for 500 ms. The subject’s task then was to decide as quickly as possible whether or not to rescue the person in the given scenario. Participants showed their decision by pressing the corresponding key (“Z” to represent rescue and “?” to represent do not rescue) on the keyboard. After the answer key was pressed, there was an intertrial interval of 1000 ms with a blank screen. The next trial started with a plus sign for 300 ms, then another name and so on. Each block contained 30 trials (each name appeared in 10 trials), and the subject took breaks between blocks. The participants’ choices (rescue, don’t rescue), reaction times (interval between presentation of the target name and the participant’s response) and ERP while viewing the names and making the decision were recording. See
For each participant, during each decision trial, we recorded electroencephalograms (EEGs) continuously from 64-scalp silver/silver-chloride electrodes located according to the international 10 - 20 system. All electrodes were referenced to an electrode at the left mastoid and re-referenced off-line to another electrode at the contralateral mastoid. The horizontal electrooculogram (EOG) was recorded bipolarly from two electrodes placed 1.5 cm lateral to the left and right outer canthi, and the vertical EOG from electrodes below and above the left eye. The impedance for each electrode was kept below 5 kW. EEG signals were amplified (half-amplitude band pass 0.05 - 70 Hz) and digitized at a sampling rate of 250 Hz.
The evoked responses (ERPs) in each stimulus condition (Father, Uncle and Acquaintance) were averaged separately off-line with averaging epochs beginning 100 ms prior to presentation of the name stimulus onset and ending 500 ms after name stimulus onset (i.e., stimulus offset). Trials contaminated by eye blinks, eye movements, or muscle potentials exceeding ±50 μV were excluded from analyses. In order to test lateralization, the following 15 electrode sites were selected for statistical analysis: five left sites (F3, FC3, C3, CP3, P3; five midline sites (Fz, FCz, Cz, CPz, Pz); and five right sites (F4, FC4, C4, CP4, P4). Latencies
and peak amplitudes of N1 (90 - 130 ms) and the mean amplitude of the P2 (180 - 230 ms), N2 (290 - 340 ms) and LPC (350 - 400 ms, 400 - 450 ms, 450 - 500 ms) components were measured separately.
The frequencies of choosing to rescue the various protagonists (Father 89%; Uncle 63%; Acquaintance 11%respectively) were significantly different (
Analyzed using one-way analysis of variance (ANOVA), decision latencies differed globally across the three name stimuli, F(2, 42) = 7.34, p < 0.01. Post hoc comparisons (t values = 1.79 - 1.83, all p < 0.05, respectively) showed that the responses to fathers (445.15 ± 94.34) were faster than those to uncles (542.21 ± 61.55) or to acquaintances (551.91 ± 66.45), but latencies did not differ between uncles and acquaintances. Latency outcomes were both consistent with hypothesis and with the choice outcomes.
These results indicate that, on a behavioral level, degree of kinship affected choices to rescue or not, that is, that rescue decisions were bias to favor closer kinship relations. Note that all other test conditions were balanced except for the kinship relation portrayed (i.e., kinship varied within common scenarios).
ERPs in response to names were characterized by a negative wave at 90 - 130 ms (N1) and a positive deflection at 180 - 230 ms (P2) over the frontal-central area. These were followed by a negative wave at 290 - 340 ms (N2) over the frontal region and a long-latency positivity at 350 - 500 ms (LPC) over the central and parietal sites (see
Three-way repeated measures analysis of variance (ANOVAs) on peak amplitudes of different ERP components were conducted;, independent variables included stimulus type (Father, Uncle and Acquaintance), laterality (left, midline and right sites) and causality (front, front-central, central, central-parietal, and parietal sites). The degrees of freedom of the F-ratio were corrected according to the Greenhouse-Geisser method.
ANOVAs of the N1 amplitude and latency demonstrated no significant effects [F(2, 42) = 1.08, p > 0.05].
ANOVA on the P2 mean amplitude at 180 - 230 ms showed a significant main effect of kinship [F(2, 42) = 5.62, p < 0.01, h2 = 0.21]. Post hoc analyses further confirmed that the P2 mean amplitude tended to be larger (approaching significance) to Father than to Uncle stimuli [F(1, 21) = 3.98, p = 0.06, h2 = 0.16], significantly larger to Father than to Acquaintance [F(1, 21) = 10.55, p < 0.01, h2 = 0.33]. However, the P2 mean amplitudes were not significantly different [F (1, 21) = 2.34, p = 0.14, h2 = 0.10] between Uncle to Acquaintance conditions.
ANOVAs on the N2 average amplitudes at 290 - 340 ms showed only a significant interaction between kinship and causality [F(8, 168) = 3.97, p < 0.05, h2 = 0.16]. The differences of N2 mean amplitudes among kinship categories was significant at central-parietal [F(2, 42) = 3.23, p < 0.05] and parietal sites [F(2, 42) = 5.89, p < 0.01]. The main effect of kinship type was not significant on N2 mean amplitude [F(2, 42) = 2.38, p = 0.13, h2 = 0.10].
ANOVAs of the LPC average amplitudes at 350-400 ms showed a significant effect of kinship [F(2, 42) = 8.87, p < 0.01, h2 = 0.30], as the LPC at 350 - 400 ms
was of larger mean amplitude to Father than to Uncle [F(1, 21) = 10.62, p < 0.01, h2 = 0.34], and to Father to Acquaintance [F(1, 21) = 15.75, p < 0.01, h2 = 0.43], but not to Uncle than to Acquaintance [F(1, 21) = 1.82, p = 0.19, h2 = 0.08].
ANOVAs of the LPC average amplitudes at 400-450 ms showed a significant effect of kinship type [F(2, 42) = 12.45, p < 0.01, h2 = 0.37], as can be seen in
ANOVAs of the LPC average amplitudes at 450 - 500 ms also showed a significant effect of kinship type [F(2, 42) = 4.98, p < 0.05, h2 = 0.19]. See
To determine the timing course and order priority among different kinship, we used ERPs to examine the temporal dynamics of the kin processing evoked by the perception of visual morally laden scenarios. After verifying behaviorally the existence of kin-serving bias, specifically, lineal relative (Father) do have advantages over other relative (Uncle) and acquaintance. We also identified Such an bias can be found in both early (P2) and late processing stages (N2, LPC) in the brain, reflected by different ERP components and brain regions. More specific, our ERP results demonstrated increased P2 and LPC mean amplitudes to father stimuli, compared to those for Uncle and Acquaintance. Several previous researches found similar consistent results, such that human commonly showed kinship bias, automatically value lineal lives over other relatives or kith for the former may improve more inclusive fitness [
In the time course of cognitive process, lineal relative was significantly distinguished from other relative or kith in three main time periods (i.e., from 180 to 230 ms, from 290 to 340 ms and from 350 to 500 ms poststimulus, see
On the early visual processing, N1 (90 - 130 ms) was similar in the three conditions. It was suggested that N1 component was related to early visual attention [
After 290 ms time window, Father stimulus activated weaker N2 component in central-parietal position and greater LPC than other stimuli. Based on previous studies, decreased N2 may correlate with conflict monitoring of kin detection [
A notable late LPC (350 - 400 ms, 400 - 450 ms, 450 - 500 ms) was elicited in parietal sites by all three conditions. Father stimuli elicited a more positive ERP deflection than did Uncle and Acquaintance stimuli between 350 to 500 ms post-stimulus, and the trend lessened near 450 - 500 ms. Previous findings indicated that LPC or P3 was highly sensitive to the process of stimulus evaluation, response execution [
More interestingly, there is an obvious trend of degree effect. The order of kinship in the present research is Father, Uncle and Kith. Moreover, the pair- wise amplitude differences F-K (Father minus Kith) is greater than U-A (Uncle minus Kith) and mainly pronounced at right central and parietal sites at LPC processing stages (see
However, several limitations of the present study have to be acknowledged. One limitation of the current study is that we only adapted three names (Father, Uncle and Acquaintance) as kinship stimuli. Thus, many repetitions of the stimulus might be unavoidable, which may raise some doubt about whether subjects really make decisions every time or just repeat the same decision. Specifically, we set up scenarios just to provide an abundant judgmental situation for participants, and not aim to contrast with other non-moral judgment as control scenarios, which need to be emphasized in the future studies.
Our results provided correlated behavioral and electrophysiological evidence for the existence of kinship bias in the context of moral dilemma judgments. P2, N2 and LPC component may play sensitive indices to discriminate different kinship or stranger, which reflected psychological significance to lineal relative compared with other relatives or acquaintances. These findings may provide insights for relationship modulation or cognitive intervention among rebellious adolescents.
We thank Kevin A. Stein from Southern Utah University for editing English.
This work was supported grants from The Natural Science Foundation of China (NSFC 31660277), Guangxi Natural Science Foundation Program (2016GXNSF459, 2015GXNSFBA139136), Scientific Research Projects in Guangxi Universities (KY2015ZD078).
Zou, X. and Yan, Z.X. (2017) Sensitivity to Kinship: From Electrophysiological Perspective. Open Jour- nal of Social Sciences, 5, 70-81. https://doi.org/10.4236/jss.2017.52008