a, Virus encoding GCaMP6f (AAV-CaMKIIa-GCaMP6f) in the BLA for fiber photometry. Scale bar, 100 μm. b-d, GCaMP6f fluorescent changes in the BLA of altruistic and selfish actor mice in response to altruistic nose poke during (b) the first day of testing (‘start’, two-way RM ANOVA, group (altruistic, selfish) x time (minutes), F(1, 101)=0.86, p=0.8248; n=42 trials from 6 mice), (c) the ‘learning’ phase (two-way RM ANOVA, group x time, F(101, 14948)=3.01, p<0.00001; group, F(1, 148)=5.45, p=0.0208; n=151 trials) and (d) the last day of testing in the SDM (‘acquired’, two-way RM ANOVA, group x time, F(101, 68882)=2.46, p<0.00001; n=684 trials). e-g, GcaMP6f fluorescent changes in the BLA of altruistic and selfish actor mice in response to selfish nose poke during (e) the first day of testing (two-way RM ANOVA, group (altruistic, selfish) x time (minutes), F(101, 3939)=0.97, p=0.5541; n=42 trials from 6 mice), (f) the learning phase (two-way RM ANOVA, group x time, F(101, 56964)=2.87, p<0.00001; n=144 trials) and (g) the last day of testing in the SDM (two-way RM ANOVA, group x time, F(101, 68882)=4.25, p<0.00001; group, F(1, 564)=5.63, p=0.0179; n=566 trials). h-i, Area under the curve (AUC) after altruistic choices (0 to 5 seconds) at different periods of the SDM task in altruistic (h, one-way ANOVA, F(2, 680)=51.17, p<0.0001, first day n=25, learning phase n=117, last day n=551) and selfish (i, one-way ANOVA, F(2, 179)=5.62, p=0.0043, first day n=16, learning phase n=33, last day n=133). j-k, AUC after selfish choices in altruistic (j, one-way ANOVA, F(2, 137)=0.07, p=0.9434, first day n=28, learning phase n=48, last day n=64) and selfish (k, one-way ANOVA, F(2, 607)=0.55, p=0.5741, first day n=13, learning phase n=95, last day n=502) actor mice. l, Male mice were bilaterally injected in the BLA with AAV-CamKIIa-mCherry (control, n=7) or AAV-CamKIIa-hM4D-mCherry (hM4D, n=7). Both groups received CNO on testing day 2 and 3, 30 minutes before SDM session with familiar recipients. Number of altruistic choices in control and hM4D (two-way ANOVA RM, group (control, hM4D) x time (days 2-3), F(1, 12)=5.44, p=0.0378). m, Decision preference score in the 5 days of SDM in control (n=7) and hM4D (n=7) mice (two-way RM ANOVA, group (control, hM4D) x time (days 1-5), F(4, 48)=2.719, p=0.0404; one sample t-test to chance (0.0), control: t=10.81, df=34, p<0.0001; hM4D: t=3.17, df=34, p=0.0032). n, Number of altruistic choices on day 2-3 of the SDM in control and hM4D mice (mixed model analysis, day 2: group (control, hM4D) x time (min), F(42, 267)=0.42, p=0.9994; day 3: F(73, 685)=2.85, p<0.0001). h-k, box plots: center = median, box = quartiles, whiskers = min and max. All other values are expressed as mean ± s.e.m