相続手続・成年後見は京都【豊田司法書士事務所】へ

Whenever legal professionals or lawyers in Japan consult with aged people,  the issue regarding capacity they possess  often emerges.  This matter is the same also in the US.

In case of the US,  as applicable laws presumes that emancipated persons inherently have capacity (self-decision ability) , the opposition parties should prove their diminished capacity with "clear and convincing evidence". 
Nevertheless,  the definition of "diminished capacity" varies depending upon transactions as a matter of law, not clinical viewpoints. 

In general, according to A HANDBOOK FOR LAWYERS, the following capacity as representative example  is enumerated. 

1. Testamentary Capacity

2. Donative Capacity

3. Contractual Capacity

Among other things, above book touches on the validity of the testamentary capacity. 

Capacity is required at the time the will was executed. Thus, a testator may
lack testamentary capacity before and/or after executing a will, but if it is made during a “lucid interval,” the will remains valid.

In Japan, I've never seen such clear description related to capacity of vulnerable people. However, as a practical matter, since I also usually utilize above transaction-specific legal standards, it seems to me that there are few gaps between  the two.