It's not enough to just collect and analyze evidence.  Investigators also need a guiding theory that's sufficiently flexible to accommodate new information and sufficiently logical to show a clear pattern of cause-and-effect.  In the murder of Christine Schultz, for example, investigators surmised from lack of evidence of a break-in that the killer had a key.  That narrowed the possibilities.  The boys' descriptions also helped, although in the end, investigators adopted a theory of spousal jealousy and the evidence was forced to fit it.  In other words, they stopped using a flexible theory and relied on a rigid one that seemed logical to them.  When they did that, some of the evidence was left out.  It's important to have a theory that guides but gets discarded when evidence contradicts it.  In other words, investigators need a good grasp of how logic and science work together.

Not all death investigators are government-employed medical examiners or coroners, although their numbers are small.  One in particular has given a lot of thought to the logical principles and methods of scientific thinking upon which solving a crime is founded. Dr. Jon J. Nordby, author of Dead Reckoning: The Art of Forensic Detection, works as a forensic science consultant through his company, “Final Analysis”.  He specializes in crime scene reconstruction, evidence collection, and bloodstain pattern analysis.  He has been a consultant to the offices of several medical examiners, and once worked as a medical investigator.  Trained in forensic medicine and criminalistics, his skills developed from diverse influences, beginning with his father, an orthopedic surgeon. 

"When I was a kid," he says, "I was interested in medical diagnoses, because my dad would read x-rays and figure out exactly what the problem was.  I was fascinated that he could get those explanations from the shadows on the pictures."

However, becoming a physician was not in the cards.  Medical schools did not offer courses on how to make clear judgments, so he turned to the philosophy department.  "Philosophers were the only people interested in how explanations were produced and defended." 

From his artistic mother, Nordby also learned the value of observation.  It was natural, then, for him to follow a path that's grounded in "the ability to make observations in the broadest sense and to produce, refine, and defend rational explanations."

Nordby uses the nautical concept, dead reckoning, to clarify the primary skill that crime investigators should acquire and hone.  Sailors relied on dead reckoning when adverse conditions prevented them from using their typical navigational instruments.  From years of knowledge, skill, and experience, they acquired an instinct for guiding the ship under any conditions.

"Dead reckoning is how you figure out how to navigate when the stars aren't out and all you have is a big storm," Nordby explains. "That's the same kind of problem that I face every day at a death scene, because we have a lot of things going on.  The neat theoretical controls from the science lab are not available, so you have to know how to use your experience and reasoning to distinguish evidence from coincidence.  That's a kind of dead reckoning.  You smell the air and get a feel for whether there's perfume or gunpowder, and if so, what kind.  When you've lost all resources except your brain, you still need to be able to determine your course and position."

Part of this skill is the ability to think rationally and make distinctions among many variables in order to interpret what specific things mean at a crime scene.  In other words, you know whether you should go through a trash basket, pick up a stray thread, check beneath fresh paint, or preserve the hair on someone's brush.  You figure it out with forms of reasoning that follow certain patterns.

While most people have heard of deductive and inductive reasoning, more important to detective work is an approach called abduction.  According to Nordby, there are important distinctions to be made:

• Induction is statistical reasoning toward a probable conclusion based on the frequency of certain things occurring.  (All of John Wayne Gacy's victims found to date were male, so Gacy did not kill females.)
• Deduction is a specific conclusion restricted by the actual evidence or claims made in a chain of reasoning that leads up to it. (All of Lucas's confessions are dubious.  Lucas confessed to killing "Orange Socks."  So Long's confession about "Orange Socks" is dubious.)  It draws out something that's already contained in the claims. 
• Abduction is the process of proposing a likely explanation for an event that must then be tested.  (We think that Christine Schultz's killer may have had a key to her home.)

"Induction is the wrong way of looking at science," Nordby insists, "because the classic problem of induction is the contrary instance [something the contradicts the claim].  For example, 'All ravens are black.  This is a raven, therefore, this raven is black.'  But how do you know all ravens are black?  'Well, every raven that I've ever seen is black.'  But then you have to anticipate that one possible white raven, so that kind of reasoning is not going to work."

He believes that the true quest for science is to explain the color of a raven. "Once you have that, then you can account for them being black, but you can also account for any contrary instance.  Abduction is a way of getting an explanation that helps us understand what is and what isn't evidence."

To give an example, he points out the error of the popular understanding that crime scene investigators notice a hair or piece of fiber, and now they have their evidence.  

"Every case I've ever worked was in a filthy room.  It was full of hair and fibers.  So what are we actually looking for?  Which of all of those hairs and fibers is actually evidence?  Unless you have some notion that will make one object relevant and another irrelevant, you're going to be swamped.  In order to have something to do, you have to have something in mind.  That comes from abduction." 

Coming up with an explanation that can be tested moves the investigation and guides the accumulation of knowledge, giving way only when contradicted.  Abduction helps to make links among events, and the development of the overall theory of a crime depends on adding new links.  Abduction also keeps guessing to a minimum.

Basil Rathbone as Sherlock Holmes (CORBIS)

While Sherlock Holmes called his method deductive reasoning, in fact, he did more than that.  He would observe something about the scene and then interpret it in the context of a case.  He honed the skill of recognizing the significance of something that no one else noticed.  That means he relied on abduction first before building a chain of deductive reasoning.

It's essential that crime investigators work within a guiding frame so that certain items become evidence and certain other items can be discarded.  The better they get at the art of solid reasoning, the more efficient they'll be in solving their cases.  That means they can take on even more cases, and there's something rewarding about that.

According to Nordby, solving crimes with disciplined thinking is a rush.  In fact, Holmes became so addicted to this intellectual pleasure that he needed cocaine between cases.  Nordby knows the feeling, too.  "I get a charge out of figuring something out, under pressure, that is extremely difficult.  And it teaches me things.  I learn something new every day because the cases are all different."

Using logic to untangle chaotic death scenes and solve the complicated puzzles of crime means the difference between dead reckoning that can steer an investigator in the right direction and random guessing that can make things hopelessly confused.  Dead reckoning is an instinct for what's relevant and an ability to reason effectively from the clues.

"Correctly reading the signs," says Nordby, "is the heart of the process."

Taking evidence through reasoning to solve crimes is an important tool, but many cases prove impenetrable nonetheless.  This is especially true of past cases where modern technology was unavailable.  Today, new techniques for evidence analysis are cracking increasingly more cases, and even cold cases can be revisited from new angles.  That's exactly what the next group of forensic professionals does.