The Concept Of Crime: Criminal Codes
Criminal behaviour is defined by the laws of particular jurisdictions, and there are sometimes vast differences between and even within countries regarding what types of behaviour are prohibited. Conduct that is lawful in one country or jurisdiction may be criminal in another, and activity that amounts to a trivial infraction in one jurisdiction may constitute a serious crime elsewhere. Changing times and social attitudes may lead to changes in criminal law, so that behaviour that was once criminal may become lawful. For example, abortion, once prohibited except in the most unusual circumstances, is now lawful in many countries, as is homosexual behaviour in private between consenting adults in most Western countries, though it remains a serious offense in some parts of the world. Once criminal, suicide and attempted suicide have been removed from the scope of criminal law in some jurisdictions. Indeed, in the U.S. state of Oregon the Death with Dignity Act (passed in 1997) allows terminally ill individuals to end their lives through the use of lethal medications prescribed by a physician. Nonetheless, the general trend has been toward increasing the scope of criminal law rather than decreasing it, and it has been more common to find that statutes create new criminal offenses rather than abolishing existing ones. New technologies have given rise to new opportunities for their abuse, which has led to the creation of new legal restrictions. Just as the invention of the motor vehicle led to the development of a whole body of criminal laws designed to regulate its use, so the widening use of computers and especially the The Internet has created the need to legislate against a variety of abuses and new frauds - or old frauds committed in new ways.

Characteristics Of Offenders
Knowledge of the types of people who commit crimes is subject to one overriding limitation: it is generally based on studies of those who have been arrested, prosecuted, and convicted, and those populations—which represent only unsuccessful criminals—are not necessarily typical of the whole range of criminals. Despite that limitation, some basic facts emerge that give a reasonably accurate portrayal of those who commit crimes.

Crime is predominantly a male activity. In all criminal populations, whether of offenders passing through the courts or of those sentenced to institutions, men outnumber women by a high proportion, especially in more-serious offenses. For example, at the beginning of the 21st century, in the United States, men accounted for approximately four-fifths of all arrests and nine-tenths of arrests for homicide, and in Britain women constituted only 5 percent of the total prison population. Nevertheless, in most Western societies the incidence of recorded crime by women and the number of women in the criminal-justice system have increased. For instance, from the mid- to late 1990s in the United States, arrests of males for violent offenses declined by more than one-tenth, but corresponding figures for women increased by the same amount. To some analysts, those statistics indicated increasing criminal activity by women and suggested that the changing social role of women had led to greater opportunity and temptation to commit crime. However, other observers argued that the apparent increase in female criminality merely reflected a change in the operation of the criminal-justice system, which routinely had ignored crimes committed by women. Although arrest data suggested that female criminality had increased faster than male criminality, the national crime victim survey showed that violent offending in the United States by both males and females had fallen in the same years. At the beginning of the 21st century, the rate of murders committed by women was about two-fifths below its peak in 1980.

Age patterns
Crime also is predominantly a youthful activity. Although statistics vary between countries, involvement in minor property crime generally peaks between ages 15 and 21. Participation in more-serious crimes peaks at a later age—from the late teenage years through the 20s—and criminality tends to decline steadily after the age of 30. The evidence as to whether this pattern, widely found across time and place, is a natural effect of aging, the consequence of taking on family responsibilities, or the effect of experiencing penal measures imposed by the law for successive convictions is inconclusive. Not all types of crime are subject to decline with aging, however. Fraud, certain kinds of theft, and crimes requiring a high degree of sophistication are more likely to be committed by older men, and sudden crimes of violence, committed for emotional reasons, may occur at any age.

Social-class patterns
The relationship between social class or economic status and crime has been studied extensively. Research carried out in the United States in the 1920s and ’30s claimed to show that a higher incidence of criminality was concentrated in deprived and deteriorating neighbourhoods of large cities, and studies of penal populations revealed that levels of educational and occupational attainment were generally lower than in the wider population. Early studies of juvenile delinquents also showed a high proportion of lower-class offenders. However, later research called into question the assumption that criminality was closely associated with social origin, particularly because such studies often overlooked white-collar crime and other criminal acts committed by people of higher socioeconomic status. Self-report studies have suggested that offenses are more widespread across the social spectrum than the figures based on identified criminals would suggest.

Racial patterns
The relationship between racial or ethnic background and criminality has evoked considerable controversy. Most penal populations do contain a disproportionately high number of persons from some minority racial groups relative to their numbers in the general population. However, some criminologists have pointed out that this may be the result of the high incidence among minority racial groups of characteristics that are commonly associated with identified criminality (e.g., unemployment and low economic status) and the fact that in many cities racial minority groups inhabit areas that have traditionally had high crime rates, perhaps as a result of their shifting populations and general lack of social cohesion. Other explanations have focused on the enforcement practices of the police, which may be influenced by racial discrimination.

Detection Of Crime
In most countries the detection of crime is the responsibility of the police, though special law enforcement agencies may be responsible for the discovery of particular types of crime (e.g., customs departments may be charged with combating smuggling and related offenses). Crime detection falls into three distinguishable phases: the discovery that a crime has been committed, the identification of a suspect, and the collection of sufficient evidence to indict the suspect before a court. Many crimes are discovered and reported by persons other than the police (e.g., victims or witnesses). Certain crimes—in particular those that involve a subject’s assent, such as dealing in illicit drugs or prostitution, or those in which there may be no identifiable victim, such as obscenity—are often not discovered unless the police take active steps to determine whether they have been committed. To detect such crimes, therefore, controversial methods are sometimes required (e.g., electronic eavesdropping, surveillance, interception of communications, and infiltration of gangs).

Suspect identification
The modus operandi, or method, used by a criminal to commit an offense sometimes helps to identify the suspect, as many offenders repeatedly commit offenses in similar ways. A burglar’s method of entry into a house, the type of property stolen, or the kind of deception practiced on the victim of a fraud all may suggest who was responsible for a crime.
Visual identification of a stranger by the victim is often possible as well. The police generally present victims or witnesses who believe that they can recognize the offender with an album containing photographs of a large number of known criminals. A suspect identified in this manner is usually asked to take part in a lineup of people with similar characteristics, from which the witness is asked to pick out the suspect. However, researchers have long known that eyewitnesses often are unreliable and that most wrongful convictions have been the result of erroneous eyewitness identifications. Scholars have suggested that cross-racial identification contributes to mistaken identification, in that members of one race may have difficulty distinguishing members of another race. Likewise, post-event assimilation, the process by which witnesses incorporate new information after the incident, can significantly alter the perception of the criminal. Finally, the stress of a crime in general, and the presence of a weapon in particular, diminish the reliability of eyewitnesses as well.
In addition, such researchers have been concerned that criminal-justice officials could manipulate standard eyewitness identification procedures in order to increase the likelihood that a witness would identify a particular suspect. In the past, criminal-justice officials generally resisted implementing reforms in procedures that would increase the accuracy of the identifications, as the reformed procedures would reduce the probability that an eyewitness would make any identification at all. But the increasing accuracy of DNA evidence in the late 1990s led to considerable publicity about erroneous convictions based on standard eyewitness identification procedures, particularly in cases that resulted in a death sentence. At the beginning of the 21st century, police agencies had begun to implement the more-careful procedures that eyewitness researchers had proposed. These procedures include not encouraging witnesses to make identifications when they are unsure but instead cautioning them about the possibility of errors, making sure nonsuspects in the lineup are reasonable possibilities for identification, and having the lineup conducted by an official who does not know who the actual suspect is.

Gathering evidence
To gain a conviction in countries where the rule of law is firmly rooted, it is essential that the investigating agency gather sufficient legally admissible evidence to convince the judge or jury that the suspect is guilty. Police departments are often reasonably certain that a particular individual is responsible for a crime but may remain unable to establish guilt by legally admissible evidence. In order to secure the necessary evidence, the police employ a variety of powers and procedures. Because those powers and procedures, if exercised improperly, would enable the police to interfere with the constitutionally protected freedoms of the suspect, they are normally subject to close scrutiny by legislation or by the courts.
One important procedure is the search of a suspect’s person or property. Most common-law jurisdictions allow a search to be carried out only if there is “probable cause for believing” or “reasonable ground for suspecting” that evidence will be found. In some cases a person may be stopped on the street and searched, provided that the police officers identify themselves and state the reasons for the search.

An important aspect in the investigation of offenses is the interrogation of suspects. The aim of the questioning is usually to obtain an admission of guilt by the suspect, which would eliminate the need for a contested trial. Most countries place restrictions on the scope and methods of interrogation in order to ensure that suspects are not coerced into confessions by unacceptable means, though in practice the effectiveness of those restrictions varies greatly. In the United States, for example, suspects must be informed that they have certain rights, including the right to remain silent, to have a lawyer present during the interrogation, and to be provided with the services of a lawyer at the expense of the state if they cannot afford one. 

Fingerprinting was originally used to establish and to make readily available the criminal records of individual offenders, but it quickly came to be widely used as a means of identifying the perpetrators of particular criminal acts. Most major police forces maintain collections of fingerprints that are taken from known criminals in order to identify them later should they commit other crimes. The FBI, for example, reportedly held millions of prints in its electronic database at the beginning of the 21st century. Fingerprints found at crime scenes thus can be matched with fingerprints in such collections.
Historically, searching fingerprint collections was a time-consuming manual task, relying on various systems of classification. The development in the 1980s of computerized databases for the electronic storage and rapid searching of fingerprint collections has enabled researchers to match prints much more quickly.
Although the science of fingerprinting is popularly perceived as error-free, some critics have charged that it is not an exact science—in part because prints are rarely pristine when gathered at a crime scene—and that some defendants have been convicted on the basis of mistaken fingerprint identification.