Every society has rules about when ,where,and to whom an individual may show certain emotions.

因为when an individual may show certain emotions和where an individual may show certain emotions都不用在后面加介词，when和where在从句中做状语，而whom在从句中做宾语 ，individual may show certain emotions to 某人。。。，所以需要加介词to而且作宾语时用whom

此处when, where 和to whom作为about的宾语，用宾格形式。