Being a noted authority on the history of the Italian ocean liner Andrea Doria, I have been asked this same question over a thousand times. I have spent over 25 years looking into this question and am 100 percent positive with the following conclusions. Here is a quick synopsis of my highly controversial answer:
There were many small errors made by both ships, the Italian liner Andrea Doria and the Swedish-American liner Stockholm, that contributed to the collision. Each small error concatenated to become bigger errors. However, the biggest error that fateful evening of July 25th, 1956 was the misreading of the radar by the young Third Officer Carstens aboard the Stockholm. This was truly the fatal error and was pivotal for the collision to have taken place that foggy evening at 11:10 pm!
As far as formal guilt for the collision, there was none that was legally attributed. There were pre-trial depositions that were taken, in contemplation of a full trial, but both ship owners, deciding that it would not be in their best interests to have a long, drawn-out trial, settled their differences with their common insurers. Therefore-it never went to trial to decide who was at fault. Informally, many professional mariners in the US went over all the testimony; and understanding the laws of the sea figured that, barring perjury from either the Doria or Stockholm's officers and crew, the Stockholm bridge officer was mistaken about the radar. These mariners have always believed that the radar misreading was the critical error. Every decision that Stockholm's young and inexperienced Third Officer Carstens made on the bridge from 20 minutes before the collision to the collision time suggest that he thought that the Doria was much further away. It made the most reasonable and logical sense for this theory. However, Carstens has vehemently denied this, but computer projections of the collision prove out, with very little doubt that the reason for this collision was the radar misreading by Carstens.
This misread of the radar by Carstens has overwhelming and uniform acceptance; and is supported by the world's most prestigious mariners/nautical scientists. Making this type of error is a very simple thing to do on a busy bridge with fog and a vessel closing fast. Back in those days, the radar rings were not illuminated and the radar's range scale selection was done manually by moving a switch. It is believed by these experts that Carstens thought that he was on a different range (15 mile) when in reality he was on another range (5 mile). In order to verify what range you are on in the night, a small flashlight is used. All the decisions that Carstens made on the bridge would support that Carstens believed that he was on a distant range and that the Doria was farther away then he thought.
For fair balance, a good account of the collision with a totally different interpretation is the book Collision Course by Alvin Moscow. In his book, Mr. Moscow attributed the blame to the captain of the Andrea Doria-Piero Calamai. For many years, Moscow's book was the definitive source for the collision. A marine engineer named John Carrouthers who was contracted by the Navy to investigate marine accidents concluded otherwise and could prove with utmost certainty that the accident was caused by the Stockholm. As the world's nautical experts had an opportunity to look at all the evidence, they have felt that the misreading of the radar was highly probable as the fatal mistake. Currently, at the United States Merchant Marine Academy (USMMA), the cadets use this accident scenario every semester as an example of radar misreading on the bridge. The textbook used for this course is called Watchstanding Guide for the Merchant Officer by Captain Robert J. Meurn Publisher: Cornell Maritime Press, ISBN 0-87033-409-3. (May 1990; updated 2006) In this book, Captain Meurn spends several pages (pp173-176) explaining the reasons for the collision of the Doria-Stockholm on misreading of the ship's radar by the Stockholm and uses this as a learning tool to make sure that any nautical/maritime officer avoids this ship-to-ship encounter. Additionally all cadets are taught how to man a ship's bridge using a multimillion dollar simulator that has the complete accident of the collision programmed specifically with what was occurring out at sea on July 25th, 1956. Captain Meurn is a master mariner and professor emeritus at the USMMA and this textbook is used at the United States Coast Guard and the US Naval Academies; as well as for NOAA mariners.
Since there was never a formal trial (just pre-trial deposition) the ultimate truth will always be left up to the individual; however, with the 50th anniversary of the collision and sinking approaching, there is no doubt that professional mariners in the United States know who was at fault. Coming late Spring 2006, look for a new book by Andrea Doria survivor Pierette Simpson who will provide new and startling evidence for the facts behind the collision and sinking of the Andrea Doria.