Complex sentence: The probability of help has often appeared to be inversely related to the number of bystanders; in other words, the greater the number of bystanders, the less likely it is that any one of them will help.
Split into simpler sentences: The probability of help appears to be inversely related to the number of bystanders. In other words, the greater the number of bystanders, the less likely it is that any one of them will help.