 If she fails to detect real interest in the woman, and thus, fails to share this female interest with her son, then it is more costly to her than if she falsely detects sexual interest from a woman towards her son and encourages him to pursue.If her son misses an opportunity, he has missed the chance to pass on his, and in doing so her own, genes.Therefore, the mother shows the same overperception bias as her son, favouring the false positive error because it is less costly.