When two businesses join together, either by merging or by one company taking over the other. There are three sorts of mergers between firms: horizontal integration, in which two similar firms tie the knot; vertical integration, in which two firms at different stages in the supply chain get together; and diversification, when two companies with nothing in common jump into bed. These can be a voluntary marriage of equals; a voluntary takeover of one firm by another; or a hostile takeover, in which the management of the target firm resists the advances of the buyer but is eventually forced to accept a deal by its current owners. For reasons that are not at all clear, merger activity generally happens in waves. One possible explanation is that when share prices are low, many firms have a market capitalization that is low relative to the value of their assets. This makes them attractive to buyers (see tobin). In theory, the different sorts of mergers have different sorts of potential benefits. However, the damning lesson of merger waves stretching back over the past 50 years is that, with one big ex ception – the spate of leveraged buy-outs in the United States during the 1980s – they have often failed to deliver benefits that justify the costs.