What determines if a soldier is gifted with psionic powers?
I just reloaded my game about 10 times trying to figure this out and I have some results. I tested the same 5 colonels, randomly mixing them together in testing, every single one of them was psi gifted, but not every time. It is not determined when the character is created, the fact that the colonel with the lowest will became gifted but only once out of the 10 times points to randomly gifting the soldiers being tested but adding their will score to the roll.
Simply put, if you want your favorite characters to all be psychics it is easy, it will just take a long time, you will have to put them through testing by themselves or with rookies added in and keep reloading until they become psychic then work on the next character. I have gotten 2 out of the 3 as gifted in about half of the 10 tests, but I was not able to garner a 3 for 3.
Did a couple of experiments that netted about 8 psi soldiers so far: 1 Col, 3 Maj, 1 Lt, 3 Squaddies. 2/3 psi soldiers everytime as I lack the patience. UPDATE: I managed to get 3/3 psi soldiers in a session: 2 squaddies and 1 major.
It requires about 4 savegames max and a lot of patience. Firstly before putting any soldiers in the psi labs, create savegame#1. Then put in 3 soldiers then create savegame#2. Wait for the 10 days ignoring any UFOs or missions.
Note down if the soldiers are gifted, if not go back to savegame#1 and pick another set of soldiers and overwrite savegame#2 until you get a hit.
Once you found the gifted soldier load savegame#2, replace the rest and create savegame#3 then wait again for 10 days and reload savegame#2 rinse and repeat until you got a hit. So you'll have 2/3 of your soldiers guaranteed to be psi gifted.
I got squaddies with ~50 will to become psi soldiers and I only test the assaults, support and heavy classes, snipers when I am actually bored.
This way I am not writing off majors and colonels with 90 and above will.
Conclusion: the soldiers are randomly selected to be psi gifted when they are entered into the psi lab. So in theory it is possible to get at least 60% psi soldiers with this method.
Hope this helps.
Psionic Soldiers 100%
Make 1 save for back up in case you mess up. Make 1 save for testing the soldier.
The game determines if a soldier has psionic power the momment of testing. Attributes might give them a higher rate of success but I'm not really sure. Every soldier can be gifted with enough chances. Patience is needed though.
Step 1) Save 1 file with no soldiers being tested.
Step 2) Add 3 soldiers to each psi test slot (we'll call them soldier a,b,c)
Step 3) Save under a slot 2
step 4) Skip all ufo attacks and upgrades for 10 days see if any of them are gifted. If non of the soldiers is gifted head back to "save 1" and repeat step 2. Do this until you find a gifted soldier.
step 5) You found soldier c (for example) has a gift. Load "save 2".
Step 6) Remove Soldiers a and b. Replace soldiers a and b. (This can be done with same soldiers and same order, it doesn't matter) DO NOT REMOVE THE GIFTED SOLDIERS!
Step 7) SAVE and Skip ahead 10 days recheck to see if you have anymore gifted soldiers. repeat step 6 till all 3 slot have gifted soldiers.
step 8) Once all 3 slots are filled with gifted soldier reload one last time fight, kill off ufo or skip ahead (that's up to you). After 10 days all 3 will be gifted.
P.S. can be done with one save slot but I make 2 just in case I mess up. Hope this helps GL ~link