Perception or Investigation - Dungeon Master Tools

Why open a "trapped" door yourself when a friend can
open it?

Like my previous look at Athletics or Acrobatics, I'm going to examine the difference between Perception and Investigation skill rolls in 5th Edition Dungeons and Dragons. These two checks come into play a lot during a game, and they are often ones that seems to cause some contention at the table. Usually you have characters with higher wisdom scores, like Clerics or Druids competing with higher intelligence scores like Wizards and Rogues wanting to use the skill that best suits them. Let's take a close look at the situation and see what skill is really called for.

Classic example, the adventuring party is tracking down the orcs who have abducted the village wizard and spirited her away to their overlord. As they pursue the orcs into a dense woodland, the group is staying on alert, knowing that orcs aren't completely stupid. The monsters may anticipate a tail. Sure enough a group of three orcs lie in wait to ambush the party. You ask for a Perception check from a character who is proficient. The Druid says he is and starts to roll, but the Rogue says that she has a higher Investigation skill modifier and wants to roll too. What do you do?

That dragon has one hell of a
stealth score!

In this case you can have both characters roll for their individual skills, because both apply. Let's look at each skill individually. According to the Players Handbook, Perception "lets you spot, hear or otherwise detect the presence of something. It measures your general awareness of your surroundings sand the keenness of your senses." It is directly tied to Wisdom with "reflects how attuned you are to the world around you and represents your perceptiveness and intuition". It would be safe to say that the Druid's experience of living within the woods, knowing the sounds of bird and beasts would allow him to perceive changes in the environment that may be caused by the orc rearguard. A Perception check makes sense.

The Players Handbook describes Investigation as when you "look around for clues and make deductions based on those clues." It is tied to Intelligence score which "measures mental acuity, accuracy of recall and ability to reason." So if the Rogue notices some broken branches on a cluster of shrubbery, the lack of animal sounds in the area and even the slight indentation of a boot print, she may put the pieces together to determine that someone is hiding and waiting to ambush.

You can see right away that there is a grey area here. How can the rogue pick up these clues without perceiving them? How can the Druid deduce from the signs of nature the ambush around the corner? In this case, I have no problem letting both characters make the roll. Perhaps the characters work together to piece the ambush together (if both roll well). Or you can have the Rogue roll with advantage (roll two D20s and use the higher roll) to say that the Druid was assisting her with the Investigation. Or vice versa. Like my previous post explained, the goal is to make sure you let your players do the cool things their characters can do.

Between the two of them, I think they got this mystery
solved.

Let's flip the tables a bit. Suppose the party is looting the office of a corrupt mayor looking for evidence that he is being bribed by the evil wizard. The Druid's high Perception will not help in this case, because what they are looking for is specific documents or a paper trail. The Rogue's Investigation skill will be a big help here as she pieces together the correspondence that proves the mayor is corrupt. Luckily she has the Druid along with her to keep an eye out for trouble. So when the mayor's deadly elf bodyguard tries to sneak up the stairs, the Druid rolls his high Perception skill and detects the assassin.

Another way they can work together is in detecting and disarming traps. The Druid may be able to find the discolored cobblestone on the floor. But he will need the Rogue's keen Investigation skill to put together how the trap works, and then her nimble fingers to disarm the danger.

The main idea is to keep the two skills distinct enough when possible so the players feel like they are specialized and able to use those talents when needed. It keeps the players working together to solve problems. It also lets you as the DM to build encounters that play to their strengths or prey on their weaknesses. Because sometime even heroes have a bad day.