What are the differences between judicial activism, judicial restraint, and strict constructionism?
Judicial activism, judicial restraint, and strict constructionism are all points on the spectrum on which constitutional law professors, legal scholars, judges and others stand with regard to their philosophies of how to interpret the United States Constitution.
For purposes of discussion, I'll start with strict constructionism and move leftward along the spectrum on constitutional interpretation, as where stands on this spectrum is often consistent where one stands on the broader political spectrum.
The U.S. Constitution, of course, was drafted hundreds of years ago. The emerging country that now spans a broad continent and has incorporated innumerable ethnicities, religions, cultures, and political orientations was a considerably simpler political entity in the 18th Century. That is not to say political discourse was simple, or the issues not serious; on the contrary, the political discourse of the Founding Fathers was in many ways intellectually far more stimulating than it is today, and the issues so serious that some would lead to a civil war a hundred years later. It is clear, however, that the drafters of the Constitution did not discuss many issues that are routinely part of the political discourse of the current era, issues like abortion. Consequently, constitutional scholars and judges have to think in terms of "what would the Founding Fathers have thought" when considering case law today. That, needless to say, leads to disagreements.
Strict constructionists hold firm to the belief that the Constitution should be interpreted literally, and that legal constructs must follow directly from the explicit wording of that document. Adherents of this philosophy disdain scholars and legal activists who they believe expand the meaning of constitutional provisions beyond what the document's authors could possibly have intended. As such, strict constructionism is the polar opposite of judicial activism.
Judicial restraint can be placed to the right of the center of the spectrum. Followers of this legal philosophy argue that a literal interpretation of the Constitution is not feasible given the unique set of circumstances that existed when it was written. More so than the other categories discussed here, practioners of judicial restraint rely on existing case law in arriving at decisions, and they tend to look more closely at congressional intent in interpreting the constitutionality of laws. Not as extreme as strict constructionism, it is nevertheless far closer to that end of the spectrum than to the next category to be covered, judicial activism.
Richard Nixon, in campaigning for the presidency in 1968, stated that, if elected, he would appoint judges who were "strict constructionists" rather than judicial activists, which was his, and many other coservatives' view of the Supreme Court lead by Chief Justice Earl Warren. Judicial activism is a political lightening rod because its practitioners are willing to go much further than constructionists or judicial restraint theorists in interpreting constitutional intent and are far more willing to make law based upon looser interpretations of the Constitution reflecting the contemporary era and to set legal precedents.
While I agree with much of the previous answer, I do not think that it is really accurate to say that strict constructionism can be placed along a continuum with judicial activism and judicial restraint. Instead, strict constructionism is really on a different continuum with loose construction being the other pole. A person can be a strict constructionist and believe in judicial restraint or a strict constructionist and believe in judicial activism.
For example, we can look at the recent case about “Obamacare” to see this dynamic in action. Strict constructionists generally argued that Obamacare was not justifiable under the commerce clause of the Constitution. They said that the power to regulate commerce did not include the power to compel people to buy insurance. Loose constructionists argued the opposite.
However, some of the strict constructionists argued for judicial activism while one seemed to argue for judicial restraint. Judicial activism supports the idea that judges should be quick to overturn actions of the elected branches that seem unconstitutional to them. Four of the conservative justices voted to overturn Obamacare. They can be said to have been acting in accordance with judicial activism. By contrast, Chief Justice Roberts could have been said to be acting based on judicial restraint. Though he took a strict constructionist reading of the Commerce Clause, he did not vote to overturn the law. Instead he found that the law was justifiable under the right to tax. Thus, he did not want to substitute the judgment of the Court for that of the elected branches if at all possible.
Thus, we see that people who believed in strict constructionism could also believe in judicial activism or in judicial review. This shows that those ideologies exist on a different continuum than the strict constructionism-loose constructionism continuum.