The amount of serious crime reported in the United States during 1976 showed no increase the first time since 1972 that the nation's annual crime rate didn't go up.

Statistics compiled by the FBI show that, in various categories of violent crime, the number of reported incidents actually decreased. Murder and robbery each dropped 10 per cent, aggravated assault went down 1 per cent, and the number of forcible rapes remained the same as in 1975.

These are four of the seven categories measured by the FBI for its annual statistical index, the Uniform Crime Reports. Preliminary figures for 1976, being released today, show an increase in only one catergory: larceny-theft which rose 5 per cent. Of the remaining two, motor vehicle thefts dropped 6 per cent and burglaries 5 per cent.

The figures indicated a substantial drop in the crime rate curve that began rising in 1972 and reached 18 per cent in 1974 - the biggest annual increase in the 47 years that the FBI has been compiling the Uniform Crime Reports. In 1975, the rise was 10 per cent over the previous year.

Although the causes of increased crime during the early 1970s were unclear, many analysts suspect that they were related to rising unemployment and to the growth of the younger segment of the population - those aged 25 or under - which accounts for the bulk of arrests.

Justice Department and other law enforcement officials said yesterday that it is similarly difficult to pinpoint the reasons for the more recent drop in the crime rate. Several paid it probably was due to a combination of factors whose interrelationship and relative importance are impossible to measure.

Among the reasons suggested by these sources were a greater public awareness of crime that has made people more careful about safeguarding their persons and property, a vastly expanded use of private security forces and devices by industry, retail businesses and apartment houses, and increased efforts in the schools to instill a respect for the law in younger people.

However, the sources also pointed out, there is no way of telling whether the drop in the crime rate will last. They noted that such factors as a new increase in unemployment could cause a new upsurge of crime.

In addiiton, several sources said, there is continuing controversy in law enforcement circles about whether the FBI statistics give a valid picture of the national crime situation. The FBI figures are based on the number of crimes reported by 9,160 state and local police agencies; and because they do not include unreported incidents of crime, some critics charge that they give a distorted and inaccurate picture of the situation.

The FBI's breakdown of crime on a geographic basis shows that the biggest inbcrease during 1976 was in the Northeastern states, where crime increased 5 per cent. The Western states were up 1 per cent, while crime declined 3 per cent in the Middle West and 1 per cent in the South.

Urban areas, traditionally the biggest centers of crime, generally showed no increase over th 1975 volume of crime; and 97 cities with populations of more than 100,000 reported decreases during 1976. Rural areas reported an overall increase of 1 per cent.

In the District of Columbia, the total reported offenses inthe seven categories for 1976 were 49,726, as compared to 55,157 in 1975. The District also showed a drop in each of the seven categories: murder 188 (235 in 1975), forcible rape 508 (520 in 1975), robbery 7,044 (9137 in 1975), aggravated assault 2,659 (2812 in 1975), burglary 11,869 (13,164 in 1975), larceny-theft 24,506 (25,892 in 1975) and motor vehicle theft 2,952 (3,397 in 1975). he added.

Rothstein testified before the Sub-committee on Criminal Justice which is considering several proposals for a federal compensation program. One plan calls for spending $40 million for fiscal year 1978, $50 million in 1979 and $60 million in 1980.

Similar crime compensation legislation was cleared by the House Judiciary Committee during the last Congress but was never enacted.