The behaviour is defined at the relationship between the entities. Go to Settings-Customization-Customize Entities-Account and go to 1:N Relationships. Select the relationship of interest (you'll have to go to the individual activity types), change them parental to configurable cascading and modify as required and publish.
Leon Tribe
Want to hear me talk about all things CRM? Check out my blog
http://leontribe.blogspot.com/
or hear me tweet @leontribe
Want to hear me talk about all things CRM? Check out my blog
http://leontribe.blogspot.com/ or hear me tweet @leontribe