The stigmata are supposed to be the spontaneous appearance of the crucifixion wounds of Jesus on a person’s body. The wounds are alleged to be the same as those that occurred on Jesus’ body, including the wounds in the hands, feet, side, scalp (crown of thorns), the back (from being whipped), and face (being beaten). The person who has the stigmata may have only one or all of the wounds.
Typically, the stigmata happen with Roman Catholics near Catholic holidays dealing with the person of Jesus, i.e., his triumphal entry into Jerusalem, crucifixion date, resurrection, etc. But, the Roman Catholic Church has reported stigmata at different times of the year on different people.
Are the stigmata effects real? There is no doubt that different people have spontaneously manifested the wounds that would have appeared in the body of Christ due to his crucifixion. But whether or not these are signs from God, demonic manifestations, or psychological manifestations of the person is hotly debated.
Since the stigmata happen within the Roman Catholic context most of all (I've not heard of it occurring with non-Roman Catholics) and since Roman Catholicism is an apostate religion, it is doubtful that the stigmata are legitimate occurrences from God. Furthermore, in the real crucifixion of Christ the nails in the hands were not in the palms but the wrists since they considered the wrists to be part of the hand. But we see in the stigmata manifested in Roman Catholics that the wounds occur in the palms and not in the wrists. If the stigmata were real, then they should manifest real physical locations that correspond to the actual wounds of Christ.
Finally, since Roman Catholicism is so filled with superstition, it is probable that the majority of the stigmata that occur with Roman Catholics are the result of self-induced psychological fervor. The human brain is very powerful, and there are numerous accounts throughout the medical field of physical manifestations in the body due to emotional excitement and belief. So, it's with great reservation that anyone should consider the stigmata to be legitimate.