How come some Parent records don't inherit activities?

[Inspector]

I'm having a hard time understanding relationships vis a vis when a Parent record displays a Child's activities in the Related List section and when it doesn't.

It seems that any Activities associated with a Contact will always display in the Account record.

However, I have a custom object that is a Child to Accounts, and in that case, the Child Activities do NOT display in the Account Related Fields List section.

Can someone explain to me why this is so, when on the Fields Setup Page, the relationships between these Child records and the Account record look exactly the same?

[Jia Hu]

Based on my understanding, Related List can only display the fields on the direct Child object on the Parent record page.

How can you display Contact' activities in the Related List of Account directly.

I mean not by adding custom fields or buttons on the Contact object to display activity.

[Jia Hu · Accepted solution]

I guess you are using Lookup relation. If you use Master-Detail it will work. At that time, when you create a activity, system will add the parent account'id into this activity. Therefore, the activity you created is also a child of account. Then it can be displayed in the account's list.